Thessalhydra (CR 15)

Huge Aberration
Alignment: Always neutral
Initiative: +4; Senses: all-around vision, darkvision 60 ft., Listen +4, and Spot +29

AC: 32 (-2 size, +24 natural), touch 8, flat-footed 32
Hit Dice: 18d8 (225 hp)
Fort +14, Ref +6, Will +15
Speed: 30 ft.
Space: 15 ft./15 ft.
Base Attack +13; Grapple +34
Attack: bite +24 melee and 8 bites +22 and tail pincer +22
Full Attack: bite +24 melee and 8 bites +22 and tail pincer +22
Damage: bite 4d6+13/19-20 plus 4d6 acid, bite 1d6+6/19-20 plus poison, tail pincer 2d6+6/19-20
Special Attacks/Actions: Breath weapon, swallow whole
Abilities: Str 36, Dex 10, Con 27, Int 5, Wis 14, Cha 11
Special Qualities: fast healing 10, Immune acid, poison
Feats: Ability Focus (breath weapon); Alertness; Improved Critical (bite, tail pincer); Improved Initiative; Iron Will; Multiattack
Skills: Listen +4, Search +1, and Spot +29
Advancement: 19-26 HD (Huge), 27-46 HD (Gargantuan), 47-58 HD (Colossal)
Climate/Terrain: Any swamp
Organization: Solitary, pair, or nest (3-6)
Treasure/Possessions: None

Source: Dungeon #134

Acid (Ex): A thessalhydra's primary bite attack drips with potent acid. A character bitten by a thessalhydra must make a DC 27 Reflex saving throw, or the damage caused by the acid also applies to any armor or robes worn. The save DC is Constitution-based.

All-Around Vision (Ex): A thessalhydra's multiple snake heads constantly peer in every direction. This grants the monster a +4 racial bonus on Search and Spot checks. A thessalhydra can't be flanked.

Breath Weapon (Su): Once every 1d4+1 rounds, a thessalhydra may spit a glob of acid. This glob has a range of 30 feet, and affects all targets in a 10-foot-radius spread. Creatures in this area take 12d6 points of acid damage, or half if they make a DC 29 Reflex save. If a character fails the save, 1d4 of his items (determine which items are affected using theItems Affected by Magical Attacks table) suffer this damage as well, although the PC may attempt a new DC 29 Reflex save for each item affected to halve the damage done. The save DC is Constitution-based, and includes a +2 bonus for Ability Focus.

Improved Grab (Ex): To use this ability, the thessalhydra must hit with its primary bite attack or its tail pincer attack. It can then attempt to start a grapple as a free action without provoking an attack of opportunity. If it wins the grapple check, it establishes a hold and can try to swallow the opponent in the following round.

Poison (Ex): Injury, Fortitude DC 27, initial and secondary damage 1d6 Strength damage. The save DC is Constitution-based.

Swallow Whole (Ex): A thessalhydra can try to swallow a grabbed opponent of Large or smaller size by making a successful grapple check. Once inside, the opponent takes 2d6+13 points of bludgeoning damage plus 15 points of acid damage per round from the thessalhydra's gizzard. A swallowed creature can cut its way out by using a light slashing or piercing weapon to deal 25 points of damage to the gizzard (AC 22). Once the creature exits, muscular action closes the hole; another swallowed creature must cut its own way out. A Huge thessalhydra's interior can hold 1 Large, 2 Medium, 8 Small, 32 Tiny, or 128 Diminutive or smaller opponents.