Mikaeus, the Unhallowed + Altar of Dementia + Soulstinger

Mikaeus, the Unhallowed

Altar of Dementia

Soulstinger

How it works

1. Activate Altar of Dementia by sacrificing Soulstinger.
2. When Soulstinger dies, its undying ability triggers, returning it from your graveyard to the battlefield with a +1/+1 counter on it.
3. When Soulstinger enters the battlefield, it triggers, putting a -1/-1 counter on it.
4. Resolve the Altar of Dementia ability
5. Repeat.

This loop turns Altar of Dementia into an endless mill machine by exploiting the counter-canceling rules of Magic. Whenever Soulstinger enters the battlefield, Mikaeus, the Unhallowed grants it undying, while Soulstinger's own ability places two -1/-1 counters on itself. State-based actions immediately cause those -1/-1 counters and the +1/+1 counter from undying to annihilate each other, leaving the creature with a clean slate to be sacrificed to Altar of Dementia all over again.