One of my favorites...though the challenge rating?
Sea Serpent, Shipbreaker
Colossal Dragon (Aquatic)
Hit Dice: 38d12+383 (630 hp)
Initiative: +4
Speed: 80 ft. (16 squares), swim 240 ft.
Armor Class: 44 (-8 size, +42 natural), touch 2, flat-footed 44
Base Attack/Grapple: +38/+74
Attack: Bite +51 melee (4d6+30 plus poison)
Full Attack: Bite +51 melee (4d6+30 plus poison)
Space/Reach: 130 ft./85 ft. or 60 ft./40 ft. (Coiled)
Special Attacks: Constrict 4d6+30, improved grab, poison, swallow whole
Special Qualities: Coiled form, darkvision 60 ft., immunity to sleep and paralysis, scent, slender form
Saves: Fort +33, Ref +23, Will +26
Abilities: Str 50, Dex 11, Con 30, Int 11, Wis 17, Cha 21
Skills: Hide +25, Intimidate +25, Knowledge (nature) +2, Listen +44, Move Silently +20, Search +40, Spot +44, Survival +24, Swim +50
Feats: Ability Focus (poison), Blind-Fight, Cleave, Endurance, Fast Swim (b), Great Cleave, Great Fortitude, Improved Grapple, Improved Initiative, Iron Will, Lightning Reflexes, Power Attack, Toughness, Weapon Focus (bite)
Environment: Any aquatic
Organization: Solitary
Challenge Rating: 27
Treasure: Half coin, double goods, standard items
Alignment: Usually chaotic neutral
Advancement: 39 + HD (Colossal +)
Level Adjustment: -
It's body scales are dark gray-brown, festooned with barnacles, seaweed, and other sea life. It's maw is the size of a large wagon, with teeth the size of greatswords.
A shipbreaker is thought to be the largest of the sea serpents, a true behemoth that rules the seas. It is a fearless hunter that enjoys attacking the largest seagoing vessels and crushing them in it's mighty coils. Fortunately the beast spends most of it's time hibernating. It is most often sighted deep at sea, usually just before it approaches and breaks up a naval vessel into kindling.
A shipbreaker sea serpent is 152 feet long and 19 feet thick. The beast has broad flippers 38 feet long and 25 feet wide that it can use to help propel it's bulk onto land. The shipbreaker sea serpent weighs 152 tons (304,000 pounds).
It is unknown what language the shipbreaker speaks or understands, if any, since none have ever reported successfully speaking with it.
Combat
A shipbreaker sea serpent attacks ships and huge-sized creatures as it's primary prey. It ignores smaller creatures except as snacks. It's favorite tactic is to ambush a ship r approach it at speed, then wrap it in it's coils and crush it. It then feasts on the sailors at it's leisure.
Coiled Form (Ex): A shipbreaker sea serpent can make itself smaller by coiling itself into a smaller space. This reduces it's effective size in combat by one size category. This does not change it's ability scores or reduce it's beneficial size modifiers. This only reduces it's detrimental size modifiers. Although it's space and reach while coiled are included in the statistics above, the other variables of this maneuver are not included in the statistics above.
Constrict (Ex): A shipbreaker sea serpent, on a successful grapple check, deals 4d6+30 points of damage.
Improved Grab (Ex): A shipbreaker sea serpent must hit an opponent up to one size smaller with it's bite attack to use this ability. It can then start a grapple check as a free action without provoking an attack of opportunity. If it wins the grapple check, it establishes a hold and can try to constrict or swallow the foe whole in the following round.
Poison (Ex): A shipbreaker sea serpent's bite is lethally poisonous. Fortitude save (DC 41), 3d6/3d6 Constitution damage. The save DC is Constitution-based, and includes a +2 Ability Focus bonus.
Slender Form (Ex): A shipbreaker sea serpent is exceptionally slender for a creature of it's incredible length. This means that it is in truth one size category smaller than it's length would indicate.
Swallow (Ex): A shipbreaker sea serpent can try to swallow Gargantuan or smaller creatures or objects by making a successful grapple check. The swallowed creature or object takes 2d8+30 points of crushing damage per round plus 2d8 points of acid damage per round from the shipbreaker's gizzard. A swallowed creature can cut it's way out by using claws or a light slashing weapon to deal 40 points of damage to the gizzard (AC 31). Once the creature exits, muscular action closes the hole; another swallowed creature must cut it's own way out.
A shipbreaker sea serpent's interior can hold one Gargantuan, two Huge, four Large, sixteen Medium, 64 Small, or several hundred Tiny or smaller creatures or objects.
Skills: A shipbreaker sea serpent has a +8 racial bonus on swim checks to perform a special action or avoid a hazard. It can always take a 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.