Workhorse + Mikaeus, the Unhallowed + Altar of Dementia

Workhorse

Mikaeus, the Unhallowed

Altar of Dementia

How it works

1. Activate Workhorse by removing all +1/+1 counters, adding that many C.
2. Activate Altar of Dementia by sacrificing Workhorse.
3. Workhorse undying ability triggers, returning it to the battlefield with an additional +1/+1 counter on it.
4. Resolve the Altar of Dementia ability, causing target player to mill one card.
5. Repeat.

This loop relies on the constant rebirth of Workhorse, whose native ability to strip away its own counters leaves it completely vulnerable to dying. That vulnerability becomes its greatest strength thanks to Mikaeus, the Unhallowed, whose static undying ability ensures that whenever Workhorse hits the graveyard, it bounces right back to the battlefield with a fresh +1/+1 counter ready to be converted into colorless mana. Meanwhile, Altar of Dementia capitalizes on this endless cycle by turning each sacrifice into incremental self-mill, letting you grind through your deck while generating infinite mana and death triggers.