Mikaeus, the Unhallowed + Altar of Dementia + Thunderblust

Mikaeus, the Unhallowed

Altar of Dementia

Thunderblust

How it works

1. Activate Altar of Dementia by sacrificing Thunderblust.
2. When Thunderblust 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 Thunderblust.
5. When Thunderblust 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 loop relies on a brilliant counter-cancellation quirk between Thunderblust's native persist mechanic and the undying ability granted to it by Mikaeus, the Unhallowed. Because Thunderblust enters with a -1/-1 counter from persist and a +1/+1 counter from undying on alternate iterations, state-based actions cause these counters to annihilate each other, resetting the creature so it can trigger either ability all over again. Meanwhile, Altar of Dementia harnesses this endless cycle by turning each sacrifice into fuel, milling away an opponent's entire library one iteration at a time.