Resolve the Witch Engine ability, adding ({B} magic symbol) ({B} magic symbol) ({B} magic symbol) ({B} magic symbol) and not giving control of Witch Engine to an opponent due to it no longer being on the battlefield.
Cast Corpse Dance with buyback by paying ({4} magic symbol) ({B} magic symbol) , returning Witch Engine from your graveyard to the battlefield with haste, and returning Corpse Dance to your hand.