Kromo, Perpetual Hydra

Kromo enters the battlefield with X +1/+1 counters on it.
Whenever a spell or ability allows you to take extra turns, put two +1/+1 counters on Kromo.
Remove two +1/+1 counters from Kromo: Regenerate Kromo.
Remove eight +1/+1 counters from Kromo: Take an extra turn after this one. Only activate this ability anytime you can cast a sorcery and only if your devotion to green is greater than eight.