Mikaeus, the Unhallowed + Phyrexian Altar + Soulstinger

Mikaeus, the Unhallowed

Phyrexian Altar

Soulstinger

How it works

1. Activate Phyrexian Altar 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. Repeat.

This loop relies on the clever interplay between Mikaeus, the Unhallowed and Soulstinger to continually reset their counters while generating an infinite supply of mana through Phyrexian Altar. Whenever Soulstinger is sacrificed to the Altar, Mikaeus's undying ability brings it back with a +1/+1 counter, which is immediately canceled out by the two -1/-1 counters placed upon it via Soulstinger's own entry trigger. Because these counters annihilate each other through state-based actions, Soulstinger returns to the battlefield completely clean of counters, ready to be sacrificed to the Altar all over again.