Mikaeus, the Unhallowed + Defiant Greatmaw + Phyrexian Altar

Mikaeus, the Unhallowed

Defiant Greatmaw

Phyrexian Altar

How it works

1. Activate Phyrexian Altar by sacrificing Defiant Greatmaw, adding one mana of any color.
2. When Defiant Greatmaw dies, its undying ability triggers, returning it from your graveyard to the battlefield with a +1/+1 counter on it.
3. When Defiant Greatmaw enters the battlefield, its first ability triggers, putting two -1/-1 counters on it.
4. Defiant Greatmaw's second ability triggers, removing a -1/-1 counter from another creature you control.
5. Repeat.

This infinite mana loop is masterminded by Mikaeus, the Unhallowed, whose static ability grants undying to all your non-Human creatures, ensuring they return to the battlefield when they perish. You sacrifice Defiant Greatmaw to Phyrexian Altar to generate mana and send the Greatmaw to the graveyard, from whence Mikaeus's granted undying returns it with a +1/+1 counter. When Defiant Greatmaw re-enters, its own triggered ability forces it to swallow two -1/+1 counters—which immediately annihilates the +1/+1 counter it just returned with via state-based actions, leaving it clean and ready to be sacrificed to the Altar all over again.