Blightbeetle + Harmless Offering + Phantom Train

Blightbeetle

Harmless Offering

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 turns Blightbeetle's passive hate into the engine for an infinite sacrifice machine by giving the beetle away to your opponent with Harmless Offering. Because Blightbeetle states that opponents' creatures can't have +1/+1 counters placed on them, any creature you sacrifice to Phantom Train's activation cost that would normally return with a counter—such as an undying creature—is instead denied that counter. This restriction prevents the safety valve that would normally stop the loop, allowing you to feed the same creature to Phantom Train endlessly to generate infinite enter-the-battlefield, leave-the-battlefield, and death triggers.