Leviathan (CR 25)

Colossal Magical Beast
Alignment: Usually neutral
Initiative: -2 (Dex); Senses: darkvision 60 ft., low-light vision, Listen +21, and Spot +20

AC: 22 (-8 size, -2 Dex, +22 natural), touch 0, flat-footed 22
Hit Dice: 32d10+329 (505 hp); DR: 10/-
Fort +28, Ref +16, Will +13
Speed: Swim 90 ft.
Space: 50 ft./15 ft.
Base Attack +32; Grapple +63
Attack: Bite +39 melee, or 2 tail slams +39 melee
Full Attack: Bite +39 melee, or 2 tail slams +39 melee
Damage: Bite 4d6+15 and gulp, tail slam 2d6+15
Special Attacks/Actions: Gulp, ramming, swamping
Abilities: Str 40, Dex 7, Con 30, Int 4, Wis 13, Cha 5
Special Qualities: SR 36
Feats: Alertness; Cleave; Great Cleave; Diehard; Endurance; Improved Critical (bite); Iron Will; Power Attack; Toughness x3
Skills: Listen +21, Spot +20, and Swim +23
Advancement: 33-48 HD (Colossal)
Climate/Terrain: Any aquatic
Organization: Solitary
Treasure/Possessions: None (see text)

Source: Monster Manual II

Gulp (Ex): A leviathan can swallow a Huge or smaller creature by making a successful bite attack. The monster's interior contains a mass of surging water. Each swallowed creature must make a Swim check (DC 15) every round. Success means the swallowed creature takes 1d3 points of subdual damage but can attack the leviathan from inside with any weapon other than a bow or crossbow. Failure means the swallowed creature takes 1d3 points of subdual damage and must immediately make a second Swim check (DC 15) to avoid going underwater and starting to drown. Anyone attacking the leviathan from the inside hits automatically. If the leviathan takes 50 points of damage from the inside, it disgorges the entire contents of its stomach.

Ramming (Ex): Once every 12 rounds, a leviathan can move at up to 720 feet and ram a Large or smaller creature or any sort of ship. To ram, the leviathan must end its movement in the target's space. If the target is a ship, the creature always precedes this attack with a long surface approach so everyone aboard can see what's coming. This attack deals 8d6+22 points of damage. If the target is a creature, it can attempt either an attack of opportunity or a Reflex save (DC 31) for half damage.

Upon ramming a ship, the leviathan can make a Strength check to breach its hull, which causes it to sink in 1d10 minutes. The break DC varies with the type of vessel rammed, as follows: rowboat DC 20, keelboat DC 23, sailing ship or long-ship DC 25, warship DC 27, or galley DC 30. (See Ships for more information.) Regardless of the check result, every creature aboard must attempt a Reflex saving throw (DC 15). Success means the creature takes 1d10 points of damage from being thrown about by the impact; failure means the creature is hurled overboard.

Swamping (Ex): Once every 12 rounds, a leviathan can create waves up to 40 feet high by rising from the water and then slamming its enormous tail flukes or head against the surface. This causes any sailing vessel within 300 feet to capsize if the character steering it fails a Profession (sailor) check (DC 15). A modifier applies to this check based on the type of ship, as follows: rowboat -2, warship +0, galley or keelboat +3, sailing ship +5, longship +7. Any creature flung into the water by a capsizing ship must succeed at a Swim check (DC 15) or immediately begin drowning.

Obviously, a creature such as the leviathan has no natural enemies. Everything in the ocean is its prey, but ships seem to be the only things that catch its attention. Once it breaches or swamps a ship, the leviathan circles back and begins scooping the bodies of both the living and the dead into its 20-foot-wide mouth. Its tactics imply a level of anger and a desire to spread destruction that contradict its low Intelligence score.