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 damage per head. A successful Reflex save halves the damage. The save DC is 10 + 1/2 hydra's original number of heads + the hydra's Constitution modifier. 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.

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. A severed head regrows in about a month.