Veinwitch Coven, Carmen, Cruel Skymarcher, and Phyrexian Altar

Prerequisites

• Veinwitch Coven on the battlefield
• Carmen, Cruel Skymarcher on the battlefield
• Phyrexian Altar on the battlefield

Steps

1. Activate Phyrexian Altar by sacrificing the one-drop creature, adding
2. When that creature dies, it and Carmen trigger
3. Resolve the creature's ability, creating a creature token
4. Resolve the Carmen ability, gaining 1 life and putting a +1/+1 counter on Carmen
5. When you gain life, Veinwitch Coven triggers, causing you to pay to return the one-drop creature from your graveyard to your hand
6. Activate Phyrexian Altar by sacrificing the token creature, adding one colored mana
7. Cast the creature by paying one mana
8. Repeat

Results

• Infinite +1/+1 counters on a creature
• Infinite creature ETB
• Infinite creature LTB
• Infinite creature sacrifice triggers
• Infinite death triggers
• Infinite lifegain