Thalia and The Gitrog Monster, Royal Assassin, and Thornbite Staff

Prerequisites

• Thalia and The Gitrog Monster on the battlefield
• Royal Assassin on the battlefield
• Thornbite Staff on the battlefield
• Royal Assassin does not have summoning sickness
• Thornbite Staff attached to Royal Assassin

Steps

1. Whenever a creature enters the battlefield under an opponent's control, it enters tapped due to Thalia and the Gitrog Monster
2. Holding priority, activate Royal Assassin by tapping it, destroying the creature
3. When the creature dies, Royal Assassin triggers, untapping Royal Assassin
4. Repeat

Results

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