Hydras are reptile-like monsters with multiple heads. A hydra is gray-brown to dark brown, with a light yellow or tan underbelly. The eyes are amber and the teeth are yellow-white. It is about 20 feet long and weighs about 4,000 pounds. Hydras do not speak.

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 make a successful sunder attempt with a slashing weapon. (The player should declare where the attack is aimed before making the attack roll.) Making a sunder attempt provokes an attack of opportunity unless the foe has the Improved Sunder feat. An opponent can strike at a hydra?s heads from any position in which he could strike at the hydra itself, because the hydra?s head writhe and whip about in combat. An opponent can ready an action to attempt to sunder a hydra?s head when the creature bites at him. Each of a hydra?s heads has hit points equal to the creature?s full normal hit point total, divided by its original number of heads. Losing a head deals damage to the body equal to half the head?s full normal hit points. A natural reflex seals the neck shut to prevent further blood loss. A hydra can no longer attack with a severed head but takes no other penalties.
Each time a head is severed, two new heads spring from the stump in 1d4 rounds. A 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 heads, at least 5 points of fire or acid damage must be dealt to the stump (a touch attack to hit) before the new heads appear. A flaming weapon (or similar effect) deals its energy damage to the stump in the same blow in which a head is severed. Fire or acid damage from an area effect may burn multiple stumps in addition to dealing damage to the hydra?s body. A hydra does not die from losing its heads until all its heads have been cut off and the stumps seared by fire or acid.
A hydra?s body can be slain just like any other creature?s, but hydras possess fast healing (see below) and are difficult to defeat in this fashion. Any attack that is not (or cannot be) an attempt to sunder a head affects the body.
Targeted magical effects cannot sever a hydra?s heads (and thus must be directed at the body) unless they deal slashing damage and could be used to make sunder attempts.
Fast Healing (Ex): Each round, a hydra heals damage equal to 10 + the number of its original heads.
Skills: Hydras have a +2 racial bonus on Listen and Spot checks, thanks to their multiple heads.
A hydra has a +8 racial bonus on any Swim check to perform some special action or avoid a hazard. It can always choose to take 10 on a Swim check, even if distracted or endangered. It can use the run action while swimming, provided it swims in a straight line.
Feats: A hydra?s Combat Reflexes feat allows it to use all its heads for attacks of opportunity.

Huge Magical Beast (Fire)
These reddish hydras can breathe jets of fire 10 feet high, 10 feet wide, and 20 feet long. All heads breathe once every 1d4 rounds. Each jet deals 3d6 points of fire damage per head. A successful Reflex save halves the damage. The save DC is 10 + 1/2 hydra?s original number of heads + hydra?s Conmodifier.
Fire attacks cannot prevent a pyrohydra?s stump from growing new heads (since a pyrohydra has immunity to fire), but 5 points of cold damage does.

Huge Magical Beast (Cold)
These purplish hydras can breathe jets of frost 10 feet high, 10 feet wide, and 20 feet long. All heads breathe once every 1d4 rounds. Each jet deals 3d6 points of cold damage per head. A successful Reflex save halves the damage. The save DC is 10 + 1/2 hydra?s original number of heads + hydra?s Conmodifier.
Cold attacks cannot prevent a cryohydra?s stump from growing new heads (since a cryohydra has immunity to cold), but 5 points of fire damage does.
SEE WIKIPEDIA ENTRY: Lernaean Hydra

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System Reference Document -> List of Creatures (SRD)