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

Cauldron of Souls

Corridor Monitor

Cathars' Crusade

Altar of Dementia

How it works

1. Activate Cauldron of Souls by tapping it, giving Corridor Monitor persist until end of turn.
2. Activate Altar of Dementia by sacrificing Corridor Monitor.
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. Resolve the Altar of Dementia ability from step 2, causing target player to mill cards equal to Corridor Monitor's power.
8. Repeat.

This clever loop uses Cauldron of Souls to grant persist to Corridor Monitor, setting up a self-sustaining cycle of sacrifice and rebirth. When you sacrifice the Monitor to Altar of Dementia, its persist trigger brings it right back, where its enters-the-battlefield ability untaps your Cauldron of Souls to keep the engine running. Meanwhile, Cathars' Crusade transforms those repeated returns into a compounding stream of +1/+1 counters—instantly neutralizing the -1/-1 counters from persist and growing your creatures' power so Altar of Dementia can mill your opponents out in one fell swoop.