Blunder

After an enemy lands a spell attack on you, and after they roll the damage for that attack, you can make their spell falter. The spell blows up in their face, the enemy makes a constitution saving throw to attempt to focus the spell, if they fail, they take half the damage they would have dealt, if they succeed, you take the full damage as you otherwise would.

In order to cast this spell you must be able to see the attacker.