Mairsil, the Pretender, Blood Vassal, Endling, and Barrenton Medic

Prerequisites

• Mairsil, the Pretender on the battlefield
• Blood Vassal in exile
• Endling in exile
• Barrenton Medic in exile
• Mairsil does not have a +1/+1 counter on it
• available

Steps

1. Activate Mairsil using Endling's third ability by paying , giving Mairsil undying until end of turn
2. Activate Mairsil using Blood Vassal's ability by sacrificing it, adding
3. Mairsil dies, triggering its undying ability, returning it from your graveyard to the battlefield with a +1/+1 counter on it
4. Activate Mairsil using Barrenton Medic's second ability by putting a -1/-1 counter on it, untapping it
5. Repeat

Results

• Infinite black mana
• Infinite death triggers
• Infinite ETB
• Infinite LTB
• Infinite sacrifice triggers