Retribution of the Ancients, Phyrexian Altar, and Butcher Ghoul

Prerequisites

• Retribution of the Ancients on the battlefield
• Phyrexian Altar on the battlefield
• Butcher Ghoul on the battlefield
• Butcher Ghoul has no +1/+1 counters on it

Steps

1. Activate Phyrexian Altar by sacrificing Butcher Ghoul, adding
2. Butcher Ghoul dies, triggering its undying ability, returning it from your graveyard to the battlefield with a +1/+1 counter on it
3. Activate Retribution of the Ancients by paying and removing a +1/+1 counter from Butcher Ghoul, giving any creature -1/-1 until end of turn
4. Repeat

Results

• Infinite death triggers
• Infinite ETB
• Infinite LTB
• Infinite sacrifice triggers
• Reduce any number of target creatures' toughness to zero