Games26

Enfeeble 

Enfeeble is a simply amazing debuff against enemy right clickers, but is easily overlooked. Use it on the enemy hard carry or anyone else whose main damage comes from their right clicks. It's blocked by Black King Bar, but if you get it off before they activate their Black King Bar then the effect won't be dispelled. Lasts 20 seconds, which is huge.

How should this spell be used correctly and in what situations?

Basically to take an enemy out of combat for a few seconds to focus on other enemies or on yourself/allies to save them with the temporary 1 second invulnerability. You can also deny yourself/allies with it.

Additionally, Nightmare can be used to allow your team to get in position for a gank and/or to set up some form of disable (especially unreliable stuns, such as  Sacred Arrow or Split Earth).