Mikaeus, the Unhallowed + Crystalline Crawler + Phyrexian Altar

Mikaeus, the Unhallowed

Crystalline Crawler

Phyrexian Altar

How it works

1. If Crystalline Crawler has a +1/+1 counter on it, activate its first ability by removing a +1/+1 counter from it, adding one mana of any color.
2. Repeat step 1 until Crystalline Crawler has no +1/+1 counters on it.
3. Activate Phyrexian Altar by sacrificing Crystalline Crawler, adding one mana of any color.
4. Crystalline Crawler dies, triggering its undying ability, returning it to the battlefield with a +1/+1 counter on it.
5. Repeat.