Grave Guardian

Flying
Whenever a creature enters the battlefield from a graveyard under your control, it becomes an
artifact in addition to it's other types.
It has "{t}: Sacrifice this creature, add {b} to your
mana pool."
{p/b}{p/b}{p/b}, {t}, Sacrifice a creature: Return target creature with converted mana cost X from a graveyard to the battlefield tapped under your control, Where X is the sacrificed card's
toughness.