Blightbeetle + Phyrexian Altar + Stiltzkin, Moogle Merchant

Blightbeetle

Phyrexian Altar

Stiltzkin, Moogle Merchant

How it works

1. Donate Blightbeetle to an opponent
2. Activate Phyrexian 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 fascinating loop relies on giving your opponent a Blightbeetle, whose static ability ensures that creatures they control can never receive +1/+1 counters. By continuously donating an undying creature with Stiltzkin, Moogle Merchant across the table into your opponent's territory, Blightbeetle's continuous effect shuts down the counter that would normally prevent that creature from returning from the grave. You can then feed that endlessly recurring creature into Phyrexian Altar to churn out infinite colored mana and death triggers, fueling your entire engine.