Subservience to greater power is the only law in the pit.

Demonic Taskmaster

Flying
At the beginning of your upkeep, sacrifice a creature other than Demonic Taskmaster.

Innistrad Remastered

Avacyn Restored

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.

If you control no other creatures, you won’t sacrifice anything.