Mikaeus, the Unhallowed + Altar of Dementia + Heartmender

Mikaeus, the Unhallowed

Altar of Dementia

Heartmender

How it works

1. Activate Altar of Dementia 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 Altar of Dementia ability.
4. Activate Altar of Dementia 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 Altar of Dementia ability.
7. Repeat.