Nagahydra (CR 18)

Huge Aberration
Alignment: Usually neutral
Initiative: +2 (Dex); Senses: darkvision 60 ft., scent, Listen +21, and Spot +21
Languages: Common and one of the following: Abyssal, Celestial, or Infernal

AC: 20 (-2 size, +2 Dex, +10 natural), touch 10, flat-footed 18
Hit Dice: 18d8+90 (174 hp)
Fort +11, Ref +10, Will +17
Speed: 40 ft., swim 50 ft.
Space: 5 ft./10 ft.
Base Attack +13; Grapple +31
Attack: Bite +22 melee
Full Attack: 7 bites +22 melee
Damage: Bite 2d6+10 plus poison
Special Attacks/Actions: Poison, spells
Abilities: Str 30, Dex 14, Con 20, Int 16, Wis 19, Cha 21
Special Qualities: fast healing 15
Feats: Alertness; Combat Casting; Combat Reflexes; Dodge; Eschew Materials; Iron Will; Lightning Reflexes; Toughness; Weapon Focus (bite).
Skills: Bluff +18, Concentration +18, Diplomacy +9, Disguise +18 (+20 acting), Intimidate +21, Listen +21, Sense Motive +17, Spellcraft +16, Spot +21, and Swim +18
Advancement: 19-36 HD (Gargantuan); 37-54 HD (Colossal)
Climate/Terrain: Temperate forests
Organization: Solitary, pair, or nest (3-4)
Treasure/Possessions: Standard

Source: Serpent Kingdoms

Like a hydra, a nagahydra can attack-with all its heads at no penalty, even if it moves or charges during the round.

A nagahydra can be killed either by severing all its heads or by slaying its body. To sever a head, an opponent must make a successful sunder attempt with a slashing weapon. (The player should declare where the attack is aimed before making the attack roll.) Making a sunder attempt provokes an attack of opportunity unless the foe has the Improved Sunder feat. An opponent can strike at a nagahydra's heads from any position in which he could strike at the nagahydra itself because its heads writhe and whip about in combat. An opponent can ready an action to attempt to sunder a nagahydra's head when the creature bites at him.

Each of a nagahydra's heads has hit points equal to the creature's full normal hit point total, divided by 5. Since a normal nagahydra has 174 hit points, 34 or more points of damage severs ahead (174 + 5 34.8, rounded down to 34). A natural reflex seals the neck shut to prevent further blood loss. A nagahydra can no longer attack with a severed head but takes no other penalties. A severed nagahydra head grows back from a stump in 1d4 rounds. The application of at least 5 points of acid or fire damage to a stump (touch attack required to hit) prevents a stump from regrowing for 24 hours. A flaming weapon (or similar effect) deals its energy damage to the stump in the same blow in which a head is severed. Fire or acid damage from an area effect (such as a fireball or dragon breath) may burn multiple stumps in addition to dealing damage to the nagahydra's body. A nagahydra does not die from losing its heads until all of them have been cut off and the stumps seared by fire or acid.

A nagahydra's body can be slain just like any other creature's, but nagahydras possess fast healing and thus are difficult to defeat in this fashion. Any attack that is not (or cannot be) an attempt to sunder a head affects the body. For example, area effects deal damage to a nagahydra's body, not to its heads. Targeted magical effects cannot sever a nagahydra's heads (and thus must be directed at the body) unless they deal slashing damage and could be used to make sunder attempts.

Poison (Ex): Bite, injury, Fortitude DC 24, initial and secondary damage 1d12 Con. The save DC is Constitution-based.

Spells: Nagahydras cast spells as 15th-level sorcerers, and can also cast spells from the cleric list and from the Destruction and Scalykind domains. A nagahydra can cast one spell per round per head, but all spells come from the common pool of spells available.

Typical Sorcerer Spells Known (6/8/7/7/7/7/6/4; save DC 15 + spell level): 0- detect magic, detect poison, light, mage hand, open close, prestidigitation, ray of frost, read magic, resistance; 1st- alarm, burning hands, mage armor, ray of enfeeblement, Tenser's floating disk; 2nd- bull's strength, eagle's splendor, fog cloud, scorching ray, see invisibility; 3rd- blink, dispel magic, fireball, fly; 4th- confusion, fire shield, stoneskin, wall of fire; 5th- cone of cold, dominate humanoid, summon monster V, teleport; 6th- chain lightning, globe of invulnerability, mass hold person; 7th- forcecage, limited wish.

Skills: A nagahydra has a +2 racial bonus on Listen and Spot checks, thanks to its multiple heads. A nagahydra has a +8 racial bonus on any Swim check to perform some special action or avoid a hazard. It can always choose to take 10 on a Swim check, even if distracted or endangered. It can use the run action while swimming, provided it swims in a straight line.

Feats: A nagahydra's Combat Reflexes feat allows it to use all its heads for attacks of opportunity.