Blightbeetle + Zedruu the Greathearted + Phantom Train

Blightbeetle

Zedruu the Greathearted

Phantom Train

How it works

1. Donate Blightbeetle to an opponent
2. Activate Phantom Train 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 Phantom Train ability.
5. Repeat from step 2.

By using Zedruu the Greathearted to donate Blightbeetle to an opponent, you establish a lock on their side of the board that prevents their creatures from receiving +1/+1 counters. With this static restriction in place, Phantom Train can repeatedly sacrifice a creature with the undying mechanic; because Blightbeetle stops the counter from being placed when the creature returns, the undying trigger is never permanently suppressed. This creates an infinite loop of sacrifices, deaths, and entries that can generate endless triggers while stacking unlimited +1/+1 counters exclusively onto Phantom Train.