Hydra can reveal themselves at any time (even when knocked out).
Reveal when not knocked out: damage city by 3
Reveal when knocked out: damage city by 1
You cannot use your hero's special ability
If not knocked out, you still play cards, face up and first
If knocked out do one of the following:
Each revealed hydra agent who is knocked out damages the city by 1 if a villain survives a round.