Halo Fountain, Intruder Alarm, and Karn, Silver Golem

Prerequisites

• Halo Fountain on the battlefield
• Intruder Alarm on the battlefield
• Karn, Silver Golem on the battlefield
• undefined available

Steps

1. Activate Karn by paying , causing Halo Fountain to become an artifact creature until end of turn
2. Activate a creature you control by tapping it, adding
3. Activate Halo Fountain's first ability by paying , tapping and untapping it, creating a 1/1 Citizen creature token
4. When the Citizen enters the battlefield, Intruder Alarm triggers, untapping all creatures
5. Repeat from step 2

Results

• Infinite creature tokens
• Infinite ETB
• Infinite mana creatures you control can produce
• Infinite untap of creatures