Blightbeetle + Altar of Dementia + Blim, Comedic Genius

Blightbeetle

Altar of Dementia

Blim, Comedic Genius

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 giving away Blim, Comedic Genius to force Blightbeetle onto your opponent's side of the board, you turn Blightbeetle's passive prevention of +1/+1 counters into the ultimate engine lubricant for an undying creature. Because those counters can never be placed, any creature with undying can repeatedly exploit Altar of Dementia to mill your library into your graveyard with infinite sacrifices, entering and leaving the battlefield an endless number of times.