Royal Assassin, Urabrask the Hidden, and Thornbite Staff

Prerequisites

• Royal Assassin on the battlefield
• Urabrask the Hidden on the battlefield
• Thornbite Staff on the battlefield
• Thornbite Staff attached to Royal Assassin

Steps

1. Whenever a creature enters the battlefield under an opponent's control, it enters tapped due to Urabrask
2. Holding priority, activate Royal Assassin by tapping it, destroying the creature
3. The creature dies, triggering Royal Assassin, untapping Royal Assassin
4. Repeat

Results

• Destroy all creatures that enter the battlefield under an opponent's control