Mikaeus, the Unhallowed + Altar of Dementia + Putrid Goblin

Mikaeus, the Unhallowed

Altar of Dementia

Putrid Goblin

How it works

1. Activate Altar of Dementia by sacrificing Putrid Goblin.
2. When Putrid Goblin 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 Putrid Goblin.
5. When Putrid Goblin 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.