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When Ovinomancer enters, sacrifice it unless you return three basic lands you control to their owner's hand.
, Return Ovinomancer to its owner's hand: Destroy target creature. It can't be regenerated. That creature's controller creates a 0/1 green Sheep creature token.
Ovinomancer is a legal target for its own ability. Activating that ability will cause Ovinomancer to return to its owner’s hand, so the ability will have an illegal target. It won’t resolve, and no one will create a Sheep token.