Mikaeus, the Unhallowed + Ashcloud Phoenix + Yahenni, Undying Partisan

Mikaeus, the Unhallowed

Ashcloud Phoenix

Yahenni, Undying Partisan

How it works

1. Activate Yahenni by sacrificing Ashcloud Phoenix.
2. When Ashcloud Phoenix dies, it triggers, as well as its undying ability if it does not have a +1/+1 counter on it.
3. Resolve the first trigger, returning it from your graveyard to the battlefield face-down.
4. Resolve the second trigger, doing nothing.
5. Activate Yahenni by sacrificing Ashcloud Phoenix.
6. When Ashcloud Phoenix dies, it triggers, returning it from your graveyard to the battlefield with a +1/+1 counter on it.
7. Resolve the Yahenni abilities from steps 1 and 5.
8. Repeat.