Mikaeus, the Unhallowed + Grim Poppet + Phantom Train

Mikaeus, the Unhallowed

Grim Poppet

Phantom Train

How it works

1. Activate Phantom Train by sacrificing Grim Poppet.
2. When Grim Poppet dies, its undying ability triggers, returning it from your graveyard to the battlefield without a +1/+1 counter on it due to its replacement effect.
3. Resolve the Phantom Train ability
4. Repeat.

By utilizing Mikaeus, the Unhallowed to grant undying to Grim Poppet, you create an infinite loop of sacrifice and resurrection enabled by Phantom Train. Whenever Grim Poppet enters the battlefield, its three -1/-1 counters arrive fresh to be stripped away, allowing you to continually feed it to Phantom Train and trigger its undying ability when it dies. Because the -1/-1 counters and +1/+1 counters will continually counteract and reset with each iteration, Grim Poppet never permanently accumulates the counters that would normally stop undying from triggering.