Mikaeus, the Unhallowed + Trapjaw Kelpie + Yahenni, Undying Partisan

Mikaeus, the Unhallowed

Trapjaw Kelpie

Yahenni, Undying Partisan

How it works

1. Activate Yahenni by sacrificing Trapjaw Kelpie.
2. When Trapjaw Kelpie dies, its persist ability triggers, returning it from your graveyard to the battlefield with a -1/-1 counter on it.
3. Resolve the Yahenni ability.
4. Activate Yahenni by sacrificing Trapjaw Kelpie.
5. When Trapjaw Kelpie dies, its undying ability triggers, returning it from your graveyard to the battlefield with a +1/+1 counter on it.
6. Resolve the Yahenni ability.
7. Repeat.