Mikaeus, the Unhallowed + Twilight Shepherd + Phantom Train

Mikaeus, the Unhallowed

Twilight Shepherd

Phantom Train

How it works

1. Activate Phantom Train by sacrificing Twilight Shepherd.
2. When Twilight Shepherd 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 Twilight Shepherd.
5. When Twilight Shepherd 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.

This crafty loop hinges on Mikaeus, the Unhallowed granting undying to Twilight Shepherd, allowing the Shepherd's own native persist ability to continually alternate with undying whenever it dies. Because counters of opposite types cancel each other out upon re-entering the battlefield, Twilight Shepherd is perpetually cleansed of its previous counters every time it returns. Phantom Train acts as the engine for this continuous cycle, providing a convenient sacrifice outlet that also steadily grows larger with infinite +1/+1 counters each time the sequence repeats.