Witch Engine, causing target player to mill cards equal to
Witch Engine's power.
Activate
Witch Engine's unearth ability by paying ({2} magic symbol) ({B} magic symbol) , returning it from your graveyard to the battlefield, giving it haste and exiling it at end of turn or when it leaves the battlefield.
Activate
Witch Engine by tapping it.
Holding priority, activate
Altar of Dementia by sacrificing
Witch Engine.
Both
Witch Engine's undying ability and
Void Maw attempt to exile
Witch Engine when it would be put into your graveyard.