Frenzied Thrall

Frantic (As you cast this spell, you may exile
up to X cards from the top of your library, where
X is the amount of generic mana in this spell's
mana value. This spell costs {1} less for each
card exiled this way.)
When Frenzied Thrall enters the battlefield, target opponent loses X life,
where X is the number of cards exiled
with its frantic ability.