Death Devourer

Whenever a creature dies, you may pay
{w/b}{w/b}{b}. If you do, you gain life equal to its
converted mana cost.
Whenever you gain life, you may pay
{w/b}{w/b}{w} and lose X life. If you do, return target
creature card with converted mana cost X to
your hand.

It eats death and excretes life.