Clavicus Vile, Lord of Bargain

Whenever a creature enters the battlefield
under your control, you may search your
library for a creature card with the same
converted mana cost and put it into your
hand. At the beginning of your upkeep, you
may pay X life, where X is the converted
mana cost of the creature you searched for.
If you do, put that creature onto the
battlefield under your control.