Royal Assassin, Urabrask the Hidden, and Intruder Alarm

Prerequisites

• Royal Assassin on the battlefield
• Urabrask the Hidden on the battlefield
• Intruder Alarm on the battlefield

Steps

1. Whenever a creature enters the battlefield under an opponent's control, it enters tapped due to Urabrask and triggers Intruder Alarm
2. Holding priority, activate Royal Assassin by tapping it, destroying the creature
3. Resolve the Intruder Alarm trigger, untapping all creatures
4. Repeat

Results

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