Archelos, Lagoon Mystic, Royal Assassin, and Thornbite Staff

Prerequisites

• Archelos, Lagoon Mystic on the battlefield
• Royal Assassin on the battlefield
• Thornbite Staff on the battlefield

Steps

1. Whenever a creature enters the battlefield under an opponent's control, it enters tapped due to Archelos
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