Blightbeetle + Carrion Feeder + Zedruu the Greathearted

Blightbeetle

Carrion Feeder

Zedruu the Greathearted

How it works

1. Donate Blightbeetle to an opponent
2. Activate Carrion Feeder 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. Resolve the Carrion Feeder ability.
5. Repeat from step 2.

By using Zedruu the Greathearted to donate Blightbeetle to an opponent, you turn the beetle's passive suppression into a devastating lock for your opponents' creatures while setting up a brilliant loophole for yourself. Because Blightbeetle prevents your opponents' creatures from getting +1/+1 counters, any creature you control with undying that dies will return without its counter, neatly dodging the condition that would normally stop it from recurring infinitely. You then feed that endlessly looping creature to Carrion Feeder, turning the execution into a generator for infinite sacrifices, deaths, enters-the-battlefield triggers, leaves-the-battlefield triggers, and endless +1/+1 counters for your hungry Carrion Feeder.