Blightbeetle + Umbral Collar Zealot + Switcheroo

Blightbeetle

Umbral Collar Zealot

Switcheroo

How it works

1. Donate Blightbeetle to an opponent
2. Activate Umbral Collar Zealot 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 Umbral Collar Zealot ability.
5. Repeat from step 2.

This clever loop relies on the static text of Blightbeetle, which we hand over to our opponent using Switcheroo so its passive ability suppresses their creatures' +1/+1 counters. With that restriction locked in, Umbral Collar Zealot goes to work by repeatedly sacrificing an undying creature to fuel its ability. Because Blightbeetle prevents those counters from ever returning the creature, the undying trigger fires infinitely, generating an endless stream of sacrifice, death, and ETB triggers alongside infinite surveil.