Blightbeetle + Ashnod's Altar + Zedruu the Greathearted

Blightbeetle

Ashnod's Altar

Zedruu the Greathearted

How it works

1. Donate Blightbeetle to an opponent
2. Activate Ashnod's Altar by sacrificing the undying creature.
3. When the undying creature dies, its undying ability triggers, returning itself from your graveyard to the battlefield without a +1/+1 counter on it due to Blightbeetle.
4. Repeat from step 2.

This clever loop relies on a subtle interaction with Blightbeetle, whose static ability prevents opponents' creatures from ever receiving +1/+1 counters—negating the usual drawback that keeps undying creatures from looping indefinitely. By using Zedruu the Greathearted to hand that Blightbeetle over to an opponent, your own recurring creature can safely hit the graveyard and return to the battlefield without a counter. Once the board is set, Ashnod's Altar acts as the engine, turning this endless cycle of life and death into an infinite supply of colorless mana.