Mikaeus, the Unhallowed + Altar of Dementia + Kithkin Spellduster

Mikaeus, the Unhallowed

Altar of Dementia

Kithkin Spellduster

How it works

1. Activate Altar of Dementia by sacrificing Kithkin Spellduster.
2. When Kithkin Spellduster 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 Kithkin Spellduster.
5. When Kithkin Spellduster 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 transforms Kithkin Spellduster into an endlessly recurring mill engine by exploiting the clever interplay between counters and resurrection mechanics. When you feed Kithkin Spellduster to Altar of Dementia, its persist ability brings it back with a -1/-1 counter, but Mikaeus, the Unhallowed grants it undying, allowing the next death to instead return it with a +1/+1 counter. Because +1/+1 and -1/-1 counters actively annihilate each other through state-based actions upon entering the battlefield, Kithkin Spellduster enters clean of counters every single time it returns. This allows you to alternate back and forth between its two death triggers indefinitely, milling your opponents' entire libraries through Altar of Dementia in the process.