Mikaeus, the Unhallowed + Twilight Shepherd + Carrion Feeder

Mikaeus, the Unhallowed

Twilight Shepherd

Carrion Feeder

How it works

1. Activate Carrion Feeder 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 Carrion Feeder ability.
4. Activate Carrion Feeder 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 Carrion Feeder ability.
7. Repeat.

This ingenious loop hinges on counter-cancellation to keep Twilight Shepherd bouncing endlessly between the battlefield and the graveyard. Because Mikaeus, the Unhallowed grants undying while Twilight Shepherd naturally possesses persist, each time the Shepherd returns, its incoming counter (+1/+1 or -1/-1) immediately annihilates the preexisting one of the opposite type upon state-based actions. This leaves the card forever clean of counters and perpetually ready to die again whenever Carrion Feeder calls for a sacrifice.