Mikaeus, the Unhallowed + Canker Abomination + Phyrexian Altar

Mikaeus, the Unhallowed

Canker Abomination

Phyrexian Altar

How it works

1. Activate Phyrexian Altar by sacrificing Canker Abomination, adding one mana of any color.
2. When Canker Abomination dies, its undying ability triggers, returning it from your graveyard to the battlefield with a +1/+1 counter on it.
3. When Canker Abomination enters the battlefield, it triggers, putting a number of -1/-1 counters on it equal to the number of creatures an opponent controls.
4. If the opponent had seven or more creatures, Canker Abomination will die as a state-based action due to having 0 or less toughness. If this occurs, repeat steps 2 through 4 infinitely.
5. Otherwise, repeat from step 1.