Demonic Taskmaster
Avacyn Restored — Uncommon
Creature — Demon
Demonic Taskmaster rulings:
2012-05-01: If you control no other creatures, you won’t sacrifice anything.
2012-05-01: If Demonic Taskmaster leaves the battlefield after its ability triggers but before it resolves, you’ll still have to sacrifice a creature. Notably, if you control two Demonic Taskmasters and no other creatures, each one will force you to sacrifice the other.