Selfish Hydra

Trample, menace, affinty for Hydras (This spell
costs {1} less to cast for each Hydra you control.)
Selfish Hydra enters the battlefield with X +1/+1 counters on it.
Whenever Selfish Hydra enters the battlefield or attacks, tap all other Hydras you control.
Selfish Hydra can only attack or block alone.