Lernean Hydra, Twelve-Headed (CR 11)

Huge Magical Beast
Alignment: Usually neutral
Initiative: +1 (Dex); Senses: darkvision 60 ft., low-light vision, scent, Listen +9, and Spot +10

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AC: 15 (-2 size, +1 Dex, +6 natural), touch 9, flat-footed 14
Hit Dice: 12d10+63 (129 hp)
Fort +13, Ref +9, Will +6

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Speed: 20 ft., swim 20 ft.
Space: 15 ft./10 ft.
Base Attack +12; Grapple +26
Attack: 12 bites +17 melee
Full Attack: 12 bites +17 melee
Damage: Bite 1d10+6
Special Attacks/Actions:

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Abilities: Str 23, Dex 12, Con 20, Int 2, Wis 10, Cha 9
Special Qualities: fast healing 22
Feats: Blind-fight; Combat Reflexes; Iron Will; Toughness; Weapon Focus (bite)
Skills: Listen +9, Spot +10, and Swim +14
Advancement: -

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Climate/Terrain: Any marsh and underground
Organization: Solitary
Treasure/Possessions: 1/10 coins, 50% goods, 50% items

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Source: Monster Manual

These hydras have bodies that are immune to all attacks (but see below). 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.

Combat

Hydras can attack with all their heads at no penalty, even if they move or charge during the round.

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.