Cauldron of Souls + Corridor Monitor + Cathars' Crusade + Phyrexian Altar

Cauldron of Souls

Corridor Monitor

Cathars' Crusade

Phyrexian Altar

How it works

1. Activate Cauldron of Souls by tapping it, giving Corridor Monitor persist until end of turn.
2. Activate Phyrexian Altar by sacrificing Corridor Monitor, adding one mana of any color.
3. When Corridor Monitor dies, its persist ability triggers, returning itself from your graveyard to the battlefield with a -1/-1 counter on it.
4. When Corridor Monitor enters the battlefield, it and Cathars' Crusade trigger.
5. Resolve the Corridor Monitor trigger, untapping Cauldron of Souls.
6. Resolve the Cathars' Crusade trigger, putting a +1/+1 counter on each creature you control.
7. Repeat.

This clever loop relies on Cauldron of Souls to grant persist to Corridor Monitor, allowing it to cycle endlessly in and out of the graveyard when sacrificed to Phyrexian Altar for mana. Each time the Monitor returns with a -1/-1 counter, Cathars' Crusade steps in with a crucial +1/+1 counter for each creature you control, instantly neutralizing the persist counter through the rules of MTG state-based actions. Meanwhile, Corridor Monitor's own entrance triggers an untap of Cauldron of Souls, resetting the entire engine to run again and again.