Stalwart Successor + Safehold Elite + Yahenni, Undying Partisan

Stalwart Successor

Safehold Elite

Yahenni, Undying Partisan

How it works

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

This loop turns Yahenni, Undying Partisan's sacrifice outlet into a self-sustaining engine by exploiting the interaction between counters and persist. When you sacrifice Safehold Elite, it returns to the battlefield with a -1/-1 counter due to its persist ability, which immediately triggers Stalwart Successor to place a +1/+1 counter on it; because +1/+1 and -1/-1 counters annihilate each other as a state-based action, Safehold Elite is left with no counters at all and is completely ready to be sacrificed to Yahenni, Undying Partisan all over again.