Mikaeus, the Unhallowed + Ashcloud Phoenix + Phantom Train

Mikaeus, the Unhallowed

Ashcloud Phoenix

Phantom Train

How it works

1. Activate Phantom Train 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 Phantom Train 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 Phantom Train abilities from steps 1 and 5, putting two +1/+1 counters on it.
8. Repeat.