Stalwart Successor + Heartmender + Yahenni, Undying Partisan

Stalwart Successor

Heartmender

Yahenni, Undying Partisan

How it works

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

This clever loop turns Yahenni, Undying Partisan's sacrifice outlet into a perpetual motion machine for creature deaths and enters-the-battlefield triggers. By feeding Heartmender to Yahenni, you trigger Heartmender's persist ability to return it to the battlefield with a -1/-1 counter. That new counter instantly wakes up Stalwart Successor, whose triggered ability places a +1/+1 counter on Heartmender. In Magic's rules, the +1/+1 counter and -1/-1 counter annihilate each other as a state-based action, leaving Heartmender completely clean of counters and ready to be sacrificed to Yahenni all over again.