Encounters: Cormyrean Marshes - The Marsh of Tun
Hydra, Seven-Headed (CR 6)
Huge Magical Beast
Alignment: Usually neutral
Initiative: +1 (Dex); Senses: darkvision 60 ft., low-light vision, scent, Listen +7, and Spot +7
AC: 15 (-2 size, +1 Dex, +6 natural), touch 9, flat-footed 14
Hit Dice: 7d10+38 (77 hp)
Fort +10, Ref +6, Will +4
Speed: 20 ft., swim 10 ft.
Space: 15 ft./10 ft.
Base Attack +7; Grapple +9
Attack: 7 bites +10 melee
Full Attack: 7 bites +10 melee
Damage: Bite 1d10+4
Abilities: Str 19, Dex 12, Con 20, Int 2, Wis 10, Cha 9
Special Qualities: fast healing 17
Feats: Combat Reflexes; Iron Will; Toughness; Weapon Focus (bite)
Skills: Listen +7, Spot +7, and Swim +12
Climate/Terrain: Any marsh and underground
Treasure: 1/10 coins, 50% goods, 50% items
Source: Monster Manual
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 toll.) 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.
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.