This is pretty powerful.... Here's the thing. Almost every card in Death's arsenal wants for creatures to die. This allows you to sacrifice two creatures for one. Sure, that's a 2-for-1, but then again, if you had two graveyards out and sacrificed your Retrovirus to plague their board (creating two skeletons), then sacrificed two skeletons to kill their Firefly Queen, you have a net gain of three creatures (two skeletons and a dead FFQ), plus any fringe benefits from Condor or Bone Wall.I don't want to say it's overpowered, because it is a situational ability, but it definitely has some amazing exploit potential.