Kithriel, Steeped in Shadows

Flying
Whenever a creature dies, put a +1/+1 counter on Kithriel, Steeped in Shadows.
Whenever Kithriel attacks, you may return any number of target creature cards from defending
player's graveyard with total power less than Kithriel's to the battlefield tapped and attacking.
Exile those creatures at the end of combat or if they would leave the battlefield.