Kaervek, Void Incarnate

Whenever an opponent casts a spell, exile the top X cards
of that player’s library, where X is that spell’s mana value.
You may choose a nonland card exiled this way. Until end
of turn, you may cast that card without paying its mana
cost.

Whenever you cast a spell from exile, Kaervek deals
damage equal to that spell’s mana value to any target.

If a spell an opponent controls would be put into a graveyard, exile it instead.