There is at least one creature on the battlefield.
Steps
Cast Ovinomancer by paying ({0} magic symbol) .
Ovinomancer enters the battlefield, triggering its first ability.
Holding priority, activate Ovinomancer's second ability by tapping it and returning it from the battlefield to your hand, destroying target creature and causing its controller to create a 0/1 Sheep creature token.
Resolve the Ovinomancer trigger from step 2, choosing not to return any lands to your hand.
Repeat.
You may also repeat this loop whenever a creature enters the battlefield under an opponent's control.
Results
Destroy all creatures opponents control.
Destroy all creatures that enter the battlefield under an opponent's control.