Retribution of the Ancients, Phyrexian Altar, and Geralf's Messenger

Prerequisites

• Retribution of the Ancients on the battlefield
• Phyrexian Altar on the battlefield
• Geralf's Messenger on the battlefield
• Geralf's Messenger has no +1/+1 counters on it

Steps

1. Activate Phyrexian Altar by sacrificing Geralf's Messenger, adding
2. Geralf's Messenger dies, triggering its undying ability, returning it from your graveyard to the battlefield with a +1/+1 counter on it
3. Geralf's Messenger enters the battlefield, causing an opponent to lose 2 life
4. Activate Retribution of the Ancients by paying and removing a +1/+1 counter from Geralf's Messenger, giving any creature -1/-1 until end of turn
5. Repeat

Results

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