Mikaeus, the Unhallowed, Retribution of the Ancients, and Blood Vassal

Prerequisites

• Mikaeus, the Unhallowed on the battlefield
• Retribution of the Ancients on the battlefield
• Blood Vassal on the battlefield
• Blood Vassal does not have a +1/+1 counter on it

Steps

1. Activate Blood Vassal by sacrificing it, adding
2. Blood Vassal 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 the +1/+1 counter from Blood Vassal, targeting any creature
4. Repeat

Results

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