Stalwart Successor + Phyrexian Altar + Lingering Tormentor

Stalwart Successor

Phyrexian Altar

Lingering Tormentor

How it works

1. Activate Phyrexian Altar by sacrificing Lingering Tormentor
2. When Lingering Tormentor dies, its persist ability triggers, returning it from your graveyard to the battlefield with a -1/-1 counter on it.
3. When Lingering Tormentor enters, Stalwart Successor triggers, putting a +1/+1 counter on Lingering Tormentor
4. Repeat.

This loop turns Phyrexian Altar into an infinite mana generator by exploiting the interaction between persist and a clever replacement-style trigger. When you sacrifice Lingering Tormentor to the Altar, its persist ability brings it back with a -1/-1 counter, but Stalwart Successor immediately steps in to place a +1/+1 counter on it as the first counters put on that creature this turn. State-based actions then cause the +1/+1 and -1/+1 counters to annihilate each other, leaving Lingering Tormentor completely clean of counters and ready to be sacrificed all over again.