Vaati, Immersturm Warlord

Whenever you activate a boast ability, you may sacrifice a creature. If you do, copy that ability and you may choose new targets for the copy.
Boast — {1}{b}{r}: Return target Demon or Berserker creature card from your graveyard to the battlefield. If it's combat, that creature enters the battlefield tapped and attacking. (Activate this ability only if this creature attacked this turn.)