Blightbeetle + Altar of Dementia + Zedruu the Greathearted

Blightbeetle

Altar of Dementia

Zedruu the Greathearted

How it works

1. Donate Blightbeetle to an opponent
2. Activate Altar of Dementia 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 Altar of Dementia ability.
5. Repeat from step 2.

By leveraging Zedruu the Greathearted to donate Blightbeetle across the table, you set up a fascinating mechanical cage where your opponents' creatures are forbidden from receiving +1/+1 counters. This restriction cleanly neutralizes the intended drawback of the undying mechanic (though an unnamed undying creature is the engine's heart in the steps), allowing that creature to continuously loop from the graveyard back to the battlefield with zero counters to impede it. Whenever you feed that looping creature into Altar of Dementia, you trigger an unstoppable mill engine that grinds your opponents' libraries down to dust while generating infinite sacrifice and death triggers.