Blightbeetle + Thermopod + Stiltzkin, Moogle Merchant

Blightbeetle

Thermopod

Stiltzkin, Moogle Merchant

How it works

1. Donate Blightbeetle to an opponent
2. Activate Thermopod 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 brilliant loop relies on the passive static ability of Blightbeetle, which ensures your opponents' creatures can never receive +1/+1 counters—a crucial safety valve that neutralizes the built-in limitation of an undying creature. By using Stiltzkin, Moogle Merchant to permanently hand over control of the Blightbeetle to your target opponent, that restriction now applies directly to their side of the battlefield where your recurring creature keeps entering. With the stage set, you simply sacrifice that creature to Thermopod to generate red mana and send it to the graveyard, where its undying trigger immediately brings it right back without a counter to halt the process.