Blightbeetle + Switcheroo + Phantom Train

Blightbeetle

Switcheroo

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.

This clever loop relies on giving your opponent a Blightbeetle using Switcheroo so that its static ability—which prevents opponents' creatures from getting +1/+1 counters—actively sabotages your own engine. When you activate Phantom Train by sacrificing a creature with undying, that creature's post-mortem return is normally gated by a +1/+1 counter. However, because Blightbeetle blocks that counter from ever being placed, the undying creature comes back pristine, allowing you to feed it to Phantom Train over and over again to generate infinite death, sacrifice, and entry triggers.