Cauldron of Souls + Corridor Monitor + Cathars' Crusade + Ashnod's Altar

Cauldron of Souls

Corridor Monitor

Cathars' Crusade

Ashnod's Altar

How it works

1. Activate Cauldron of Souls by tapping it, giving Corridor Monitor persist until end of turn.
2. Activate Ashnod's Altar by sacrificing Corridor Monitor, adding CC.
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.