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

Mikaeus, the Unhallowed

River Kelpie

Yahenni, Undying Partisan

How it works

1. Activate Yahenni by sacrificing River Kelpie.
2. When River Kelpie dies, its persist ability triggers, returning it from your graveyard to the battlefield with a -1/-1 counter on it.
3. When River Kelpie enters, its first ability triggers, causing you to draw a card.
4. Resolve the Yahenni ability.
5. Activate Yahenni by sacrificing River Kelpie.
6. When River Kelpie dies, its undying ability triggers, returning it from your graveyard to the battlefield with a +1/+1 counter on it.
7. When River Kelpie enters, its first ability triggers, causing you to draw a card.
8. Resolve the Yahenni ability.
9. Repeat.