Mikaeus, the Unhallowed + Heartmender + Phantom Train

Mikaeus, the Unhallowed

Heartmender

Phantom Train

How it works

1. Activate Phantom Train by sacrificing Heartmender.
2. When Heartmender dies, its persist ability triggers, returning it from your graveyard to the battlefield with a -1/-1 counter on it.
3. Resolve the Phantom Train ability.
4. Activate Phantom Train by sacrificing Heartmender.
5. When Heartmender dies, its undying ability triggers, returning it from your graveyard to the battlefield with a +1/+1 counter on it.
6. Resolve the Phantom Train ability.
7. Repeat.