Cauldron of Souls, Corridor Monitor, Cathars' Crusade, and Phyrexian Altar

Prerequisites

• Cauldron of Souls on the battlefield
• Corridor Monitor on the battlefield
• Cathars' Crusade on the battlefield
• Phyrexian Altar on the battlefield
• Corridor Monitor does not have a -1/-1 counter on it

Steps

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

Results

• Infinite colored mana
• Infinite death triggers
• Infinite ETB
• Infinite LTB
• Infinite sacrifice triggers
• Infinite +1/+1 counters