Mikaeus, the Unhallowed + Ashnod's Altar + Pith Driller

Mikaeus, the Unhallowed

Ashnod's Altar

Pith Driller

How it works

1. Activate Ashnod's Altar by sacrifiicing Pith Driller, adding CC.
2. When Pith Driller dies, its undying ability triggers, returning it from your graveyard to the battlefield with a +1/+1 counter on it.
3. When Pith Driller enters the battlefield, it triggers, putting a -1/-1 counter on it.
4. Repeat.

This ingenious loop relies on Mikaeus, the Unhallowed to grant undying to our loop's star, Pith Driller, ensuring that every time it perishes, it bounces right back to the battlefield with a +1/+1 counter. Ashnod's Altar provides the means to sacrifice Pith Driller for a steady stream of colorless mana while simultaneously sending it to the graveyard to trigger that undying return. The magic happens because Pith Driller's own entry trigger forces it to place a -1/-1 counter on itself, which instantly annihilates the +1/+1 counter via state-based actions and leaves the creature completely clean and ready to die all over again.