What "situational" items one should get during a game and why?

 

Starting items early game

 
Clarity - Buy this if you have skills you want to spam, like Warlock's  Fatal Bonds/Shadow Word or Zeus's  Arc Lightning. Don't buy this after early game.

 
Tango - Buy this if you're expecting to be harassed by autoattacks or DoT skills early game, for example: you're a melee hero who needs to get last hits and your enemies are ranged heroes, or heroes who can harass well like Warlock. Buy Tangoes. You should usually get one stack of Tangoes anyway, but if you expect heavy harass get two stacks.

 

Healing Salve - Buy this if you're expecting to be harassed by nukers who can take out a good chunk of your health quickly, like Zeus. Do not buy this if you're expecting to get DoT harass or right click harass, as those will interrupt your salve and waste it. Get Tangoes instead.

 
Town Portal Scroll - ALWAYS have one in your inventory. Maybe two. These are super useful. They let you Town Portal in to other lanes to push, or escape from ganks (ALWAYS BUY ONE IF YOU'RE AGAINST Bloodseeker !!! So you can just Town Portal out if he ultis you and type "u mad" in all chat.)

 
Animal Courier - Always have one. The support usually buys this. If he doesn't, buy one. It's just 150 gold and it brings you items from base. Incredibly useful.

 
Flying Courier - Upgrades the Animal Courier and gives it wings so it can fly over terrain. Also gives it a speed boost ability. Make sure someone buys this asap as it helps you get items faster and also reduces the risk of the courier getting killed.

 
Observer Ward - Provides vision over an area. The support usually buys this. Try to ward the rune spots in the river early game so you can see the runes and incoming ganks.

 
Sentry Ward - Reveals invisible units and Observer Wards in an area. Use this if you think the enemy might have warded an area. E.g. you're anti mage and you decide to jungle for a bit. Suddenly you get ganked by the entire enemy team. They've obviously warded the jungle - how else would they have known? Drop a sentry ward at a common ward spot and hope you find a ward.

 
Bottle - Usually bought by the mid hero so he can Bottle runes and regen. Don't start with this item as it gives you no stat bonuses and will make it difficult to get last hits.

Mid game items

 
Dust of Appearance - Buy this if they have invis heroes so you can reveal them. Keep an eye out for enemies building Shadow Blade - dust counters them pretty well. The problem with dust is that you can't see invis units without using it, so watch out for initiators going invis and initiating on you (like Shadow Fiend).

 
Smoke of Deceit - Very useful for ganks. Buy this if you're ganking, want to initiate a teamfight or want to kill Roshan.

Late game items

Pretty much the same as mid game.

ATTRIBUTES ITEMS

Early game

 
Iron Branch - THE most cost effective attribute item in the game. Don't know what to buy with your starting gold? Get Tangoes and Iron Branches. You can't go wrong with Iron Branches. They take up lots of inventory space though, so they're only good for early game.

 
Gauntlets of Strength - Buy this if you're planning on building an Urn of Shadows or Bracer. Nice str and HP boost.

 
Slippers of Agility - Buy this if you're planning on building a Poor Man's Shield or Wraith Band. Nice agi and AS boost.

 
Mantle of Intelligence - Buy this if you're planning on building a Null Talisman. Nice int and mana boost.

 
Circlet - Pretty good stat item. Not as cost effective as Iron Branch but it takes up less space. Buy this if you're planning on building a bracer, Null Talisman or Wraith Band.

 
Belt of Strength - Buy this if you're planning on building Power Treads and you want a bit of extra str/HP before getting your Power Treads. This item also builds into Skull Basher, Necronomicon and Sange but those aren't really early game items.

 
Band of Elvenskin - Buy this if you're planning on building Power Treads and you want a bit of extra agi/AS before getting your  Power Treads. This item also builds into Yasha.

 
Robe of the Magi - Buy this if you're planning on building Power Treads and you want a bit of extra int/mana before getting your  Power Treads. This item also builds into Drum of Endurance, Blade Mail, Diffusal Blade and Oblivion Staff

Mid game

 
Ultimate Orb - Great stat item. Buy this if you're planning on building a * Linken's Sphere (spellblock, regen, damage, good for squishy carries like Weaver)

• Eye of Skadi (makes you tankier and gives you more mana as well as a great slow effect, good for DPS heroes who need survivability like Slark and also lets you kite easily)
• Scythe of Vyse (extra int/mana, mana regen, great for casters who want a disable to help them land their spells like Lina. Outworld Devourer can build a Scythe of Vyse too for the extra int.)
• Manta Style (great for right click heroes. Build this if you are a right click hero in need of more DPS, and if the enemy has single target spells like Lion or Lina so you can get them to waste their spells on your illusions.)

 
Ogre Club - Gives you extra str/hp, buy this if you are building Aghanim's Scepter, Black King Bar or Sange. More on those items later.

 
Blade of Alacrity - Extra agi/AS, buy this if you are building Aghanim's Scepter, Diffusal Blade or Yasha.

 
Staff of Wizardry - Extra int/mana, buy this if you are building Aghanim's Scepter, Dagon, Eul's Scepter of Divinity, Force Staff, Necronomicon, or Rod of Atos.

Late game

There aren't really any late game attribute items. Attribute items usually build into other items that are considered late game items.

ARMAMENTS ITEMS

Early game

Ring of Protection - The extra armour is neat, but you should only really get this if you're building a Ring of Basilius or Tranquil Boots. Both items are good in lane, more on those later.

 
Quelling Blade - Buy this if you have really low base damage and need lots of farm. ONLY BUY THIS IF YOU ARE A MELEE HERO, the bonus damage you get from it if ranged is shit, may as well get branches or something instead. This item is great if you're a jungler because you can use it to cut down trees to make safespots.

 
Stout Shield - Buy this if you are a melee carry who needs to farm and you think you'll face lots of right click harass in lane. It builds into a Poor Man's Shield or Vanguard so if you want to build either of those items, you'll need to get this.

Mid game

 
Blades of Attack - +9 damage, builds into Phase Boots, Crystalys, and Armlet of Mordiggian. Buy this if you want to build any of those items.

 
Chainmail - Buy this if you want to build a Blade Mail, Assault Cuirass, Medallion of Courage or Buckler which builds into Mekansm.

 
Quarterstaff  - Extra damage, extra AS. Buy this if you want to build an Oblivion Staff or Butterfly. Buy the Quarterstaff first if you're building a Butterfly as it will help you farm up the other items faster. If you are an int hero who is getting an Oblivion Staff for Refresher Orb or Orchid Malevolence get the other items first as they will give you more mana to cast your spells (casters don't really right click to farm anyway so the Quarterstaff isn't too useful).

 
Helm of Iron Will - Gives you bonus armour and regen, great for junglers. Buy this if you are building an Armlet of Mordiggian, Helm of the Dominator or Veil of Discord.

 
Broadsword - Extra damage. Buy this if you are building a Battle Fury, Blade Mail or Crystalys. If you're farming safely you can save up an extra 200 gold and buy a Claymore before this if you're building a Battle Fury. If you're in constant danger of being ganked, get this first.

 
Platemail - Extra 10 armour. Buy this if you're building an Assault Cuirass or Shiva's Guard.

 
Claymore - Extra damage. Buy this if building a Battle Fury or Shadow Blade.



 
Javelin - Extra damage with a chance to pierce for more damage. Buy this if building a Skull Basher or Monkey King Bar. I hate RNG so I usually get the Demon Edge first if I'm going for a Monkey King Bar, but if I'm underfarmed and in constant danger of getting ganked I usually get this first. If you're tanky enough to tank creeps, get this first if you're building a Skull Basher. If not, get the belt first for extra hp.



 
Mithril Hammer - Extra damage. Buy this if you're building a Black King Bar, Maelstrom or Desolator.

Late game

Most of the above items build into other items which are considered as lategame items, so nothing to see here.

ARCANE ITEMS

Early game

 
Magic Stick - BUY THIS IF AGAINST BATRIDER. This will save your life against a Batrider. It gains charges when he spams his Sticky Napalm, and he spams that a LOT. This will save your life, BUY IT at the start if you're up against a Batrider. You can buy it from the side shop too so if you're not sure which lane Batrider will lane in (he usually goes offlane or mid) you can buy something else at the fountain and get a few last hits in lane to buy the Magic Stick from the sideshop. Magic Stick is generally a good item against spammy heroes like Warlock, Skywrath Mage etc. But especially Batrider.

 
Sage's Mask - Buy this if you are a spammy annoying support hero who wants to build a Ring of Basilius like Warlock, Batrider etc. Gives you some nice mana regen so you can continue to
be annoyingspam your spells.

 
Ring of Regen - The +2 hp regen isn't really worth it imo, you're better off buying Tangoes early game. This item does however build into some pretty useful items like Headdress, Tranquil Boots, Vladmir's Offering and many more so if you're aiming for any of those, buy this. It won't save your ass against heavy harass in lane however.

 
Boots of Speed - PLEASE DO NOT START WITH THIS ITEM (unless you are a pro player and want to get first blood with Sand King and your trilane). It's a good item but in the early game stats and regen are more important. Builds into  Power Treads. Every hero should have boots by midgame.

 
Gloves of Haste - Builds into  Power Treads, Hand of Midas, Armlet of Mordiggian and Maelstrom. Power Treadsare useful for carries as they provide extra MS and allow you to tread switch. Hand of Midas is good for junglers, buy one if you are a carry and expect a long game full of farming.

Mid game

 
Cloak - Build this if you want to build a Hood of Defiance, good against casters like Lina.


 
Gem of True Sight - Buy this if their shadow fiend keeps ultiing you while invisible with his Shadow Blade, or you're just plain tired of their Riki running around raping everybody. Also useful if you think they've warded your jungle and you want to deward. Your team should have at least ONE gem by the end of midgame if they have invisibility heroes.

 
Morbid Mask - Lifesteal. Great for junglers as it helps you keep your hp up. Remember that it is a Unique Attack Modifier (orb effect) so it doesn't stack with other UAMs. Builds into Mask of Madness, Helm of the Dominator, and Vladmir's Offering.

 
Ghost Scepter - Great item for supports who keep getting killed by right click carries. It renders you invulnerable to autoattacks so it's a great counter to right click carries like am, weaver etc. Watch out for casters on the enemy team as they'll do more damage to you.

 
Shadow Amulet - Extra AS and lets you go invis, however the invis breaks when you move. Not very useful, and is easily countered by Dust of Appearance or Sentry Ward or any aoe stun. It doesn't interrupt channeling abilities so I think you should be able to use it while channeling a tp scroll. Builds into Shadow Blade. Good for right click carries like Sniper and drow who want to build a Shadow Blade.

 
Talisman of Evasion - 25% evasion chance. Good against right click carries, but relies on a bit of luck. Builds into Butterfly and Heaven's Halberd. Try to get the other items required to build these items before talisman, rng sucks.

Blink Dagger - Great item for initiators. Buy this item if you have a great initiation spell like Tidehunter's ravage and want to wreck teamfights. Do not buy this if you are not an initiator unless you really need that mobility (like Tinker, so he can blink into the trees and use march to split push).

UPGRADES ITEMS

COMMON

Early game

 
Null Talisman - Great for casters, gives you extra int/mana and builds into Dagon and Hood of Defiance.

 
Wraith Band - Great for agi carries, extra AS and agi and builds into Ring of Aquila.


 
Bracer - Good for heroes who need a bit more tankiness like Batrider. Extra str/hp, builds into Drum of Endurance.

 
Poor Man's Shield - Great for melee carries who are getting harassed. Especially agi carries as it boosts agi. Buy this if you are a melee agi carry getting harassed.

Mid game

 
Soul Ring - Buy this if you are a caster and run out of mana a lot like Omniknight and Bane. Generally heroes with a heal benefit the most from this as they can just heal themselves to replenish the health they lost due to the Soul Ring.

 
Phase Boots - Buy this if you are a hero who wants to chase aggressively and do damage, like Spectre or Death Prophet who can move very quickly with Phase Boots. Riskier than Power Treadsso you need to be tanky too.

 
Power Treads - Good item for carries, safer than Phase Boots and lets you tread switch. If you don't know what boots to get, Power Treads are always good.

 
Oblivion Staff - Good for casters who also want to do some right click damage like Outworld Devourer and Lina. Builds into Refresher Orbs and Orchid Malevolence. If you just want mana and int to cast spells, this item is not for you.

 
Perseverance - Great item for carries who need to farm for a long time likeAnti-Mage. Builds into Battle Fury, Linken's Sphere, Bloodstone and Refresher Orbs. Most carries pick up a Ring of Health from the sideshop to survive harass anyway, so you may as well turn that into a Perseverance later on.

 
Hand of Midas - Great item for carries that need to jungle. The attack speed bonus helps heroes with lifesteal like, er, Lifestealer. Don't buy this if the enemy is 5man pushing mid and about to end the game, this item is only for long games.

Late game

 
Boots of Travel - Buy this during the midgame if you are Tinker . If you are not Tinker, do not buy this item unless the game has gone on for a very long time, you are rolling in cash, you have no towers left and need fast tps to the creep waves so you can push.

SUPPORT ITEMS

Early game

 
Ring of Basilius - Good item if you and your laning partner are mana hungry. Gives you nice mana regen and an armour bonus. Builds into Rod of Atos and Vladmir's Offering.

 
Tranquil Boots - Good item for heroes that roam a lot and need regen, like Batrider and Lich (because Lich can restore his mana easily but can't restore his hp easily). Also gives you a bonus to MS which is nice for heroes like Batrider who need to chase.

 
Headdress - HP regen. Good for early pushes or if you're facing heavy harass. Don't get this item too early or you'll end up pushing the lane.

 
Ring of Aquila - Cheap item that makes you a bit tankier and gives you extra damage and mana regen. Pick this up if you are a carry with money to spare that needs any of the above.

 
Buckler - If you're building a Mekansm, build this first. The +2 armour buff isn't a lot but it's not bad either. It gives you extra int, agi, str and +5 armour too. Build this if you are planning on building a Mekansm.

Mid game

 
Vladmir's Offering - Buy this if you are jungling and need lifesteal to sustain yourself, or if you want to Roshan. Do not buy this if someone on your team already has one - the auras don't stack.

 
Medallion of Courage - Buy this if you want to gank early, the -armour debuff is quite significant. Good item on Visage as his passive gives him extra armour and Vengeful Spirit as it stacks with her wave of terror. This item also gives you mana regen. Do not buy this if you do not plan on ganking early.

 
Arcane Boots - At least one person on your team should have this. It's a great item - if you are a support, go for Arcane Boots, they are great boots for a support/caster. Make sure you use the active effect when your teammates are around, and make sure your arc boots are never off cooldown.

 
Drum of Endurance - Buy this if you need extra movement speed and you have right click carries on your team that need extra AS. Heroes like Ember Spirit and Spectre can become very scary chasers with a Drum of Endurance and Phase Boots. Build this if nobody on your team has one.

 
Mekansm - Heals all teammates near you for 250 hp. This item can and will save your team's life in teamfights, so make sure your support builds one. However, heroes that have been healed by Mekansm cannot be healed again for another 25 seconds, so make sure your team only builds ONE. The aura also does not stack. 1v1 heroes like Viper and Razor can build Mekansm too to solo gank the enemy.

Late game

 
Pipe of Insight - This is a great item against casters like Lina or anyone with aoe skills that do lots of damage. It creates a barrier that blocks 400 spell damage on your teammates. That's a total of 5x400 = 2000 damage to your team. A well timed Pipe of Insight can help you survive a ravage + sanity's eclipse combo that would have otherwise wiped your team. Heroes affected by barrier in the past 50 seconds cannot be affected again, so make sure you build only one.

CASTER ITEMS

Early game

There are no early game items in this category.

Mid game

 
Force Staff - This item is great on all int heroes. It gives you extra int which gives you extra mana, and also some mobility which is useful for squishy int heroes. This item also lets you Force Staff your enemy which can lead to some cheeky ganks where you stand far away from an enemy so he thinks it's safe and then Force Staff him towards you and jump on him.

 
Veil of Discord - This item gives you bonuses in all stats for a bit of survivability, and some armour and regen. The most important feature of this item is that it reduces magic resistance, making it a great item for heroes like Sand King, Tidehunter, Earthshaker and other heroes with aoe spells. The Veil of Discord actually allows epicenter to do more damage than with Aghanim's Scepter (I think, if I'm wrong please correct me) and also allows your team to do more damage. Get this if you feel like your spells aren't doing enough damage.

 
Eul's Scepter of Divinity - This is a great item for supports. It gives you extra int, mana regen and movement speed which a lot of supports desperately need (ahem Crystal Maiden ahem). Its active ability can also save you from ganks. Generally a good utility item, get this if you want to protect yourself from solo ganks (cyclone your attacker and tp out, if they have more than one attacker you're probably fucked). Heroes with hard to land stuns like Invoker (sun strike) and Lina (light salve array) benefit GREATLY from this item - cyclone your enemy, then land the stun.

 
Necronomicon - A very underrated item. This item summons two warriors that might not look like much, but they can absolutely wreck enemy casters and int heroes if not killed. The warrior burns mana on each attack - 75 mana per attack at level 3. The archer also has an active ability that burns a whopping 225 mana - however it does have a long cooldown of 20 seconds. The units also do a portion of the mana burned as damage which is neat. At level 3, the warrior has TRUE SIGHT. Never get snuck up on by a Riki ever again! And finally, the unit that kills the warrior takes 600 damage at level 3. That's quite a lot. Basically, buy the Necronomicon if you are a hero that can push and you want to push hard, if they have invis heroes, or if they have lots of int heroes. However the necro is not usually purchased due to the high cost of getting it to level 3.

 
Dagon - A simple nuke. Does an incredible 800 damage at max level but costs 7720 gold to get there. Usually purchased on nukers like Nyx Assassin and Tinker to solo gank heroes - a  Heat-Seeking Missile - Laser  - Dagon combo usually instagibs a hero. Buy this item if you are a nuker and want to one-shot your squishy enemies. However be warned that Black King Bar will block it. Make sure you have enough mana to cast the Dagon - the Dagon itself provides you with some bonus int that will increase your mana pool by a bit though.

 
Rod of Atos - A nifty little item that's great for squishy int heroes that want to chase. It gives you extra intelligence and +350 hp. Its active effect slows your target by 60% which is very significant. It also has a huge range of 1200 (same as a Blink Dagger). Buy this if you want a bit of extra mana and a lot of extra health, along with a great slow that will help you chase. Usually purchased on supports, however it is occasionally purchased by Outworld Devourer.

 
Orchid Malevolence - This is almost a late game item. Buy this if you are an int hero who can do lots of damage(Outworld Devourer!) and want to do even more damage. It gives you extra int, extra mana regen, extra damage, extra attack speed and silences your enemy for 5 seconds and amplifies the damage it takes by 30%. The silence gives you extra survivability too.

Late game

 
Aghanim's Scepter - Extra HP, extra mana, and bonuses to all stats. Only buy this item if it upgrades your ultimate. It's not top priority however, except on a few heroes like Omniknight and Zeus. You generally want to get other items before this like Black King Bar and stuff.

 
Refresher Orb - Luxury of luxury items. Get this after you've gotten everything else you need. It refreshes the cooldown on all your skills and items. Make sure you have enough mana to use it. Only buy this if you have a huge ultimate or something that will benefit from a Refresher Orbs like Outworld Devourer or Tidehunter.

 
Scythe of Vyse - Gives you extra str, agi and a lot of extra int. Increases mana regen too. The active ability turns your enemy into a sheep. What's not to love? Buy this if you are a caster who needs another disable and some extra mana, or someone who relies on stacking INT like Outworld Devourer or silencer.

WEAPONS ITEMS

Mid game

 
Crystalys - Buy this if you are a right click carry and want to
trust in rngesusget some extra damage along with the potential to crit. Try to build this into a Daedalus, it's much better and hits like a truck. Do not buy if you are a caster or support who does not right click.

 
Armlet of Mordiggian - This item is great for str heroes who want to be able to manfight better. It drains hp every second but if you have a decent hp pool you should be fine. Lifestealer works too, which is why this item is common on Lifestealer. Buy this if you have lots of hp and want to 1v1 better. Armlet of Mordiggian toggling can help you survive ganks - basically, switch off your Armlet of Mordiggian so your hp goes down, then activate it again so your hp goes back up. Keep your Armlet of Mordiggian on when your enemy attacks you, then switch it off while running away, switch it on again to tank the next attack, repeat. This does not cancel channeling. Good on heroes like Huskar and Lifestealer.

 
Skull Basher - Gives you a 25% chance to stun for 1.4 seconds. Also gives you a bit of extra str and lots of damage. Buy this if the enemy has lots of channeling abilities, or if you want to chase down enemies, or if they have high dps heroes that you want to focus down quickly (their dps items are useless if they're stunned). Only buy this if you are a melee hero.

 
Shadow Blade - Gives you extra damage and attack speed and allows you to go invis. This is useful for dps carries like drow and sniper as it gives them more DPS as well as an escape mechanism. This item is easily countered in higher-skill games by dust of appearance of sentry wards, so don't buy this unless you play in a really low mmr.

 
Battle Fury - Deals 35% of your damage as cleave damage. Only works on melee. Buy this if you are a melee hero who needs to farm quickly and has no aoe skills like Anti-Mage (I mentioned "no aoe skills" because you wouldn't want to get Battle Fury on, say, Slark for example, since he already has dark pact to do aoe damage and farm up). A fun strat is to stack bfuries on Ember Spirit and build a Daedalus too. 5 bfuries with a Daedalus will let you do crazy damage with  Sleight of Fist. Just stand back, let your teammates teamfight, and spam  Sleight of Fist. It's not uncommon to see heroes getting one-shotted by this tactic.

Late game

 
Ethereal Blade - The "shotgun" item. Converts you and your target into ethereal form, slowing your target and causing both of you to take 40% extra damage. This combined with a Tinker + maxed Dagon will instagib anyone. Ethereal Blade also causes your enemy to take 2x of your primary attribute + 75 as damage, which is why Ethereal Blade is commonly built on Morphling as his Morphling allows him to have an insanely high primary attribute. It also amplifies the damage from his spells. Building Ethereal Blade on Morphling is called the "shotgun" build because of how much burst damage it does. Buy this if you have nukes and want to instagib.

 
Radiance - Gives you +65 damage and 50 dps in a 700 radius. Buy this if you are a carry who wants more damage and wants to FARM FARM FARM. Seriously, jungling and laning is super easy with a Radiance. Some people build this on Invoker and go invis and run around. It's funny but easily countered, and if they get a gem you've just wasted 5000 gold. Don't try this unless you're in a really low MMR pub. This item is also good on heroes like doom because it stacks with his scorched earth and lets him farm a lot.

 
Monkey King Bar - Buy this if you are a carry who wants to do insane damage. BUY THIS if they have spells that decrease your accuracy like Riki's smoke, Brewmaster's Drunken Haze, or Phantom Assassin's Blur as Monkey King Bar will ensure that you maintain 100% accuracy even while under the effect of these skills.

 
Daedalus - Buy this if you believe in RNGesus and want to one-shot squishy supports. Gives you a lot of damage and a good crit. You should usually get this after another damage item like Monkey King Bar to increase the damage dealt by Daedalus' crit.

 
Butterfly - Buy this if you are an agi carry who wants more attack speed, damage, agi and evasion. The evasion is useful against right click heroes like Anti-Mage and Phantom Assassin. Get this after your other damage items, this is a luxury item. If you're really struggling to stay alive in fights go for survivability items like Black King Bar, Blade Mail, Linken's Sphere or Heart of Tarrasque.

 
Divine Rapier - The comeback item. Only purchase this if you
are not XBOCTare a right click carry who can survive fights. Only buy this if you're losing and REALLY want to make a comeback win. It's a huge gamble, but sometimes a Divine Rapier is the only way to make a comeback. Do not buy this if you are winning. Buy other things like Daedaluses (daedali?) if you are winning. I repeat, ONLY BUY THIS IF YOU ARE LOSING HARD.

 
Abyssal Blade - Gives you an extra 100 damage which is pretty insane and a bit of extra str. Its active stuns for 2 seconds through Black King Bar. Get this if you are a right click dps carry rolling in cash and want to manfight the enemy's right click carry who has a Black King Bar. This is a luxury item. Do not try to build this item first. Make sure you have lots of dps so you can kill your enemy during the 2 second stun period. It also has a passive 25% bash chance that stuns for 1.4 seconds, so make sure you have a high attack speed to take advantage of this. It does not stack with Slardar's bash, Faceless Void's Time Lock, Spirit Breaker's Greater Bash, Troll Warlord's bash, Skull Basher and other Abyssal Blade. I don't think those heroes can even purchase the item. The chance is reduced to 10% on ranged heroes, so only buy this if you are a melee hero.

ARMOR ITEMS

Mid game

 
Hood of Defiance - Gives you 8 HP regen per second and 30% magic resist. Buy this against casters like Lina so you don't get nuked to death.

 
Blade Mail - Shares damage you take to the source of the damage, i.e. you still take the damage, but your attacker does too. Does not work against magic immune units. Doesn't reflect damage from other Blade Mail. Damage is reduced by armour or magic resistance before being returned. Damage includes physical right click damage as well as spell damage. Gives you +10 int, +22 damage and +6 armour. Buy this if you are a tank like Axe who wants to tank your enemy's right click damage. Only buy this if your enemy has a lot of squishy heroes that do more damage than they can take. A well timed Blade Mail can turn a fight around very quickly.

 
Vanguard - 80% chance to block 40 damage if you are a melee hero and 20 damage if you are a ranged hero. This item peaks in the midgame but falls off quickly as the damage block quickly becomes insignificant. This item isn't very good IMO. It provides a 250+ hp boost as well as 6 hp regen - you can achieve similar bonuses with a Mekansm, plus Mekansm lets you heal your whole team. Vanguard also uses psuedo-rng which means that the damage block chance is closer to 66.7% chance. Only buy this if you have tons of extra money and don't know what to buy, or if you are axe and have lots of money.

 
Soul Booster - Gives you +450 HP, +400 mana, +4 HP regen, +100% mana regen. Build this if you are building a Bloodstone.

 
Black King Bar - One of the most useful items ingame - renders you invulnerable to spells for 10/9/8/7/6/5/4 seconds. 10 seconds of magic immunity is huge and can win a game by itself. However there are certain spells that go through Black King Bar. Keep in mind that you cannot sell a Black King Bar, you must drop it and buy a new one. Buy this if you have channelled spells that you want to cast without being stunned, if you are a carry who wants to lay down dps without being stunned, if you are a support who wants to use your spells without being stunned. Basically just buy this if the enemy has lots of stunners. Also gives you +10 str and +24 damage.

Late game

 
Shiva's Guard - Gives you +30 int and +15 armour. This item is good on heroes that want to get tankier but also get more mana (like Tidehunter) to cast their spells. It also has a good slow effect that can stack with Omniknight's passive, so it's a good item for heroes that get up in the enemy's face and need hp and mana. It is more of a luxury item however, so try not to rush for it.

 
Manta Style - +10 Strength +26 Agility +10 Intelligence +15 Attack speed +10% Movement speed. Its active produces two illusions of your hero that last 20 seconds, deal 33% damage and take 350% damage if melee, and do 28% damage and take 400% damage if ranged. Buy this if you are a right click hero who wants to do more dps and if the enemy has lots of single target spells, so they can waste them on your illusions. You are invulnerable for 0.1 seconds when you use it which lets you disjoint some projectiles if your reflexes are fast enough. Mirror image dispels negative buffs. It also gives you some nice stat bonuses which should benefit agi carries.

 
Bloodstone - Gives you +500 hp, +400 mana, +9HP regen, + 200% mana regen, +10 damage. With a Bloodstone you should be able to spam your abilities without running out of mana, ever. With a Bloodstone you will be able to respawn very quickly, and you'll also be able to deny yourself. Buy a Bloodstone if you are want to be a tank who can also spam spells (like Death Prophet) and if you want to respawn quickly. Try to be the first to die in a teamfight so you can heal your teammates, unless you're much stronger than your teammates.

 
Linken's Sphere - +15 str, +15 agi, +15 int, +6hp regen, +150% mana regen, +10 damage. It blocks most spells once every 17 seconds. This item is great on carries that need more damage and regen but also need a spell block, like Weaver, Medusa and Morphling. Weaver benefits greatly from the damage bonus as he's a dps hero and the regen because he needs to farm. The spellblock also lets him dodge hexes and stuns and escape with Shukuchi. Sorry, Weaver is just my favourite hero so I had to talk about him. Good item, do not buy if you are a support. A Black King Bar would be better.

 
Assault Cuirass - +10 armour, +35 AS, and +20 AS, +5 armour to teammates and -5 armour to enemies in an aura around you. Buy this if you are a right click melee carry who stands in the middle of the fight and wants to do lots of damage and help your team do lots of damage as well. The aura is great. The aura does not stack, so make sure your team only builds one.

 
Heart of Tarrasque - The ultimate tank item. It gives you +40str and +300 hp. It actually gives you more than 300 hp due to the str bonus. If you haven't taken damage from an enemy hero or roshan in the last 4 seconds (if melee) or 6 seconds (if ranged) you regen 2% of your total hp per second. This is some pretty insane regen, almost like a portable fountain. You can go into a teamfight, come out barely alive, regen in around 30-40 seconds, and then go back in at full health. Great item, but it is a luxury one. Buy this if you find yourself dying in teamfights even with a Black King Bar, or if you just want more tankiness. Buy this if you have good DPS - there's no point in staying alive longer if you can't do any damage during that period.

ARTIFACTS ITEMS

Mid game

 
Helm of the Dominator - Gives you a passive 15% lifesteal, +20 damage and +5 armour. Its passive allows you to take control of a non-hero, non-ancient unit. Buy this if you are a right click carry who wants more survivability from lifesteal, and if you don't have any unique attack modifiers as they will not stack with the lifesteal. You can also buy this if you are Chen and you want to raise a zoo. It builds into a Satanic which is a pretty good late game item for a DPS hero (but not Chen).

 
Mask of Madness - Gives you +100 AS and +30% MS but causes you to take +30% damage. Buy this if you are a ranged dps hero who can kite well so you don't take any damage. Faceless Void also builds this item because he barely takes any damage while in Chronosphere. Building this on melee heroes is okay as long as you're sure you won't take too much damage. If you're overfarmed and overlevelled, feel free to buy this on a melee hero. Bone7 also likes to buy this on Batrider for the extra MS bonus - it's a pretty good build but risky as Batrider often has to tank quite a bit.

 
Sange - +16 str, +10 damage. Gives a 15% chance on attack to slow AS and MS by 20% for 4 seconds. This isn't really the greatest passive - it has a low chance and doesn't do too much. If you want to build a Sange and Yasha, go for Yasha first. Buy a Sange if you want to build a Sange and Yasha.

 
Yasha - +16 agi, +15AS, +10%MS. A good item for agi carries who want to build into Sange and Yasha.

 
Sange and Yasha - A combination of the Sange and Yasha. This item used to be shit but it's quite decent now. It doesn't have a very obvious effect, but it just generally makes you hit harder and move faster, and has a slow chance too. It's sort of like having a level advantage. The components are rather cheap too so it's a good item to pick up if you're losing.

 
Desolator - +60 damage which is quite a lot. Your attacks reduce your target's armour by 7 for 15 seconds. It's a UAM which means it doesn't stack with other UAMs. Buy this if you are a dps carry who wants to do more damage to towers and enemies with high armour, especially if they have items like Shiva's Guard, Medallion of Courage and if they picked Lich. Good item to pick up if you're going for a push strat since corruption works on towers. Generally a good DPS item.

Late game

 
Mjollnir - +24 damage, +80 AS. This item is good on heroes that need to clear creep waves fast, or farm into lategame as chain lightning procs 25% of the time and does decent damage. The +80AS is no joke either. Static charge is a good active that will make your enemy think twice before focusing you down. Which they probably won't, so they'll get zapped anyway.

 
Eye of Skadi - +25 str, +25 agi, +25 int, +250hp, +250 mana. In total, Eye of Skadi gives 725HP, 575 mana, 35 damage, 25 AS, 1 mana regen/s, .75HP regen/s and 3.5 armour. That's a lot of stat bonuses and hp/mana boosts. This item makes you much more tanky and also helps you slow your enemy with Cold Attack. Cold attack is a UAM but it can be combined with a lifesteal UAM. Buy this if you are a right click carry who wants more survivability as well as the ability to chase and slow your enemy, like slark.

 
Satanic - The most insane lifesteal item ever. +25 str, +20 dmg, +5 armour, a passive 25% lifesteal, and an active effect that increases lifesteal by 175% for 3.5 seconds. This combined with a Black King Bar will turn you into a teamwiping carry. This is a luxury item, do not buy it until you've finished your other items (no point having all this lifesteal if you get stunned and die because you didn't buy a Black King Bar). Lifesteal will not activate if you miss your attack, so try to buy an Monkey King Bar too if your opponent has evasion. Buy this if you are a right click hero with insane attack speed and damage items, as well as a Black King Bar in your inventory.

SECRET SHOP

Early game

 
Orb of Venom - A cheap item that gives you a nice slow and a bit of DoT. Good for the price, get it on melee heroes who can gank early like Slark
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Ring of Health - A good item that will help you stay in lane. Pick it up on laning carries that can be harassed easily like Anti-Mage.

 
Void Stone - A good item that will help you spam spells in lane. Usually bought to finish up a Perseverance but you can also build it into a Eul's Scepter of Divinity or Scythe of Vyse if you are a support.

Mid game

 
Energy Booster - A good item that boosts your mana by 250. Pick it up if you want to build a Bloodstone or a soulbooster. Try to get the point booster instead if you can, unless you DESPERATELY need that extra mana.

 
Vitality Booster - Extra 200 hp. Pick it up if you want to build a Bloodstone or Soul Booster. Try to get thePoint Booster instead.

 
Hyperstone - Good for right click carries that want more attack speed. Builds into carry-friendly items like Assault Cuirass and Mjollnir.

 
Demon Edge - Buy this if you are a right click hero who just needs more damage. Buy this first if you are building an Monkey King Bar or Daedalus because RNGesus will forsake you when you need him most (i.e. do not buy Crystalys before Demon Edge if building an Monkey King Bar unless you already have some damage items with you.)

 
Mystic Staff - +25 int. Good for casters who need mana, good for Outworld Devourer who needs mana. Builds into Scythe of Vyse or Shiva's Guard. Only problem is that it's quite hard to buy this item since it doesn't have any components.

 
Reaver - +25 str. Buy this if you are building a heart or Satanic . If you're building a Heart of Tarrasque, try to buy this first unless you're really strapped for cash.

 
Eaglesong - Expensive item that gives you +25 agi. It doesn't have any components so it's hard to buy. Buy the Quarterstaff before this if you're building a Butterfly. Buy this if you are a right click agi hero who wants more agility, or if you're building a Butterfly or Ethereal Blade.

 
Sacred Relic - +60 damage, which is quite a lot. Builds into Divine Rapier, Radiance and Abyssal Blade. It doesn't consist of any components so it's difficult to buy. Buy this if you are a right click DPS carry who needs more damage, or if you're building any of the items it builds into. For example, Weaver, because he likes to build Radiance.

 
Aegis of the Immortal - Get it by killing Roshan. Not even going to describe it here, but let your carry take it.

 
Cheese - Restores 2500 hp and 1000 mana to its user, obtained by killing Roshan from his third respawn onwards. Give it to your carry.