Pyrohydra, Twelve-Headed (CR 13)

Huge Magical Beast (Fire)
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: Breath Weapon

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Abilities: Str 23, Dex 12, Con 20, Int 2, Wis 10, Cha 9
Special Qualities: fast healing 22, fire Immunity, vulnerable to cold
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: Warm marsh and underground
Organization: Solitary
Treasure/Possessions: 1/10 coins, 50% goods, 50% items

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Source: Monster Manual

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

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.

Fire Subtype (Ex): Fire immunity; double damage from cold except on a successful save.

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.

Fire Subtype

A creature with the fire subtype has immunity to fire. It has vulnerability to cold, which means it takes half again as much (+50%) damage as normal from cold, regardless of whether a saving throw is allowed, or if the save is a success or failure.