Eldyaux, Reaper of Souls

Lifelink
Creatures killed by Eldyaux, Reaper of Souls can't be regenerated. Eldyaux gets +1/+1 for each creature killed by it.
{x}: Play a creature your opponent controlled that Eldyaux, Reaper of Souls killed. X is equal to the amount of mana the creature cost. The ability fails if that creature is exiled, is on the battlefield, has been shuffled into its owner's deck, or you
already cast it using this ability and