Mikaeus, the Unhallowed + Altar of Dementia + Grazing Kelpie

Mikaeus, the Unhallowed

Altar of Dementia

Grazing Kelpie

How it works

1. Activate Altar of Dementia by sacrificing Grazing Kelpie.
2. When Grazing Kelpie 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 Grazing Kelpie.
5. When Grazing Kelpie 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.

This clever loop relies on a clever mechanical interaction between Mikaeus, the Unhallowed and Grazing Kelpie, where the creature's native persist ability and the granted undying ability perpetually counteract one another. When you sacrifice Grazing Kelpie to Altar of Dementia, it returns with a -1/-1 counter from persist, but Mikaeus ensures the next death triggers undying to bring it back with a +1/+1 counter that cancels the negative mark. This infinite cycle of sacrifices allows Altar of Dementia to mill your opponent's entire library away in one fell swoop.