Creestab, Progeny of Athreos

As long as your devotion to white and black is more than three each, whenever an opponent loses life, Creestab, Progeny of Athreos gets +2/+2 until end of turn and has "Creature cards in your graveyard have unearth {x}, where X is equal to it's casting cost."
"We keep the dead around until we need
them."