Hydras can attack with all their heads at no penalty, even if
they move or charge during the round.
A hydra can be killed either by severing all its heads or
by slaying its body. To sever a head, an opponent must hit the
monster's neck with a slashing weapon and deal damage equal
to the hydra's original hit point total, divided by its
original number of heads, in one blow. (The player says where the
attack is aimed just before making the attack roll.) For example,
if a five-headed hydra has 52 hp, a single blow dealing 10 or
more points of damage severs a head (52 / 5 = 10.4, rounded down
to 10). Any excess damage is lost. A severed head dies, and a
natural reflex seals the neck shut to prevent further blood loss.
The hydra can no longer attack with the severed head but suffers
no other penalties.
The only way to slay a Lernaean hydra normally is to sever
all its heads. However, each time a head is severed, two new heads
spring from the stump in 1d4 rounds. A Lernaean hydra can never
have more than twice its original number of heads at any one
time, and any extra heads it gains beyond its original number
wither and die within a day.
To prevent a severed head from growing back into two, at
least 5 points of fire or acid damage must be dealt to the stump (AC
19) before the new heads appear.
Spells such as disintegrate, finger of death, and slay living
kill a Lernaean hydra outright if they succeed. If the spell
deals damage on a successful save, that damage is directed
against one of the hydra's heads.
Skills: Hydras receive a +2 racial bonus to Listen and Spot
checks, thanks to their multiple heads.
Feats: A hydra's Combat Reflexes feat allows it to use
all its heads for attacks of opportunity each round.