Royal Assassin, Frozen Aether, and Intruder Alarm

Prerequisites

• Royal Assassin on the battlefield
• Frozen Aether on the battlefield
• Intruder Alarm on the battlefield
• Royal Assassin does not have summoning sickness

Steps

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