Avarirah, Greedy Distraction

Flying
When it enters the battlefield, each player loses 3 life and sacrifices a creature.
Whenever it deals combat damage to a player, that player mills cards from their library until a non-permanent spell is put into their graveyard. You may choose a card that was milled this way and cast it from this way. If you do, all creatures get -X/-X where X is that spell's mana cost, until end of turn.