Eternal Hydra

Hexproof
Eternal Hydra enters the battlefield with X +1/+1 counters on it.
Each time Eternal Hydra takes damage, put that many head counters on it.
At the beginning of each end step, if Eternal Hydra took damage from any source an opponent controls but didn't leave the battlefield, put a number of +1/+1 counters on Eternal Hydra equal to the number of head counters on Eternal Hydra.