Blightbeetle + Altar of Dementia + Switcheroo

Blightbeetle

Altar of Dementia

Switcheroo

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.

This elegant loop relies on locking an opponent out of counter progression while you continuously recycle a creature for parts. By using Switcheroo to hand over Blightbeetle, you saddle your target with a permanent static barrier that prevents creatures they control from ever receiving +1/+1 counters. Once that piece is settled on their side of the board, your undying creature becomes completely immune to the mechanic that would normally shut down a loop, allowing you to feed it endlessly into Altar of Dementia to mill out the entire table.