Halo Fountain + Intruder Alarm + Karn, the Great Creator

Halo Fountain

Intruder Alarm

Karn, the Great Creator

How it works

1. Activate Karn's first loyalty ability by putting a loyalty counter on it, 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.