grakmaw, skyclave ravager enters the battlefield with three +1/+1 counters on it.
whenever another creature you control dies, if it had a +1/+1 counter on it, put a +1/+1 counter on grakmaw.
when grakmaw dies, create an x/x black and green hydra creature token, where x is the number of +1/+1 counters on grakmaw.