Whisperstalker

Fear
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.)
At the beginning of each end step, if 5 or more
cards entered exile from anywhere this turn, each opponent loses 5 life.