Kuo-Toa Leviathan (CR 15)

Huge Monstrous Humanoid (Aquatic)
Alignment: Always lawful evil
Initiative: +5; Senses: darkvision 60 ft., keen sight, light blindness, Listen +17, and Spot +21

AC: 28 (-2 size, +1 Dex, +5 Wis, +14 natural), touch 14, flat-footed 28
Hit Dice: 15d8+75 (142 hp)
Fort +12, Ref +10, Will +14
Speed: 20 ft., swim 50 ft.
Space: 10 ft./10 ft.
Base Attack +15; Grapple +31
Attack: Bite +22 melee or claw +19 melee
Full Attack: Bite +22 melee and 2 claws +19 melee
Damage: Bite 2d6+8/19-20 , claw 1d6+4
Special Attacks/Actions: Improved grab, stun, swallow whole
Abilities: Str 26, Dex 13, Con 21, Int 15, Wis 20, Cha 10
Special Qualities: Amphibious, immunities, improved evasion, resistance to electricity 10, Sea Mother blessing, slippery, uncanny dodge
Feats: Great Fortitude; Improved Critical (bite); Improved Initiative; Multiattack; Power Attack; Weapon Focus (bite)
Skills: Diplomacy +2, Escape Artist +9, Hide +5, Listen +17, Move Silently +13, Search +18, Sense Motive +17, Spot +21, Survival +5 (+7 following tracks), Swim +16, and Use Rope +1 (+3 bindings)
Advancement: 16-45 HD (Huge)
Climate/Terrain: Temperate aquatic
Organization: Solitary or enclave (1 leviathan plus 2-13 kuo-toas, 2-5 kuo-toa monitors, and 2-4 kuo-toa whips)
Treasure/Possessions: Double standard

Source: Underdark

Improved Grab (Ex): If a kuo-toa leviathan hits an opponent of its own size category or smaller with its bite attack, it deals normal damage and attempts to start a grapple as a free action without provoking an attack of opportunity (grapple bonus +31). If it gets a hold, it can attempt to swallow the foe in the next round.

Stun (Ex): Five times per day, a kuo-toa leviathan can attempt to stun its opponent on an attack with one of its natural weapons. If the opponent fails a DC 22 Fortitude save, it is stunned for 1 round in addition to taking normal damage from the attack. The save DC is Wisdom-based.

Swallow Whole (Ex): A kuo-toa leviathan can try to swallow a Large or smaller opponent it has grabbed by making a successful grapple check. Once inside, the opponent takes 2d8+12 points of crushing damage plus 2d6 points of acid damage per round from the kuo-toa leviathan's digestive juices. A swallowed creature can cut its way out by using a light slashing or piercing weapon to deal at least 25 points of damage to the kuo-toa leviathan's digestive tract (AC 20). Once the creature exits, muscular action closes the hole, so another swallowed opponent must cut its own way out.

A kuo-toa leviathan's interior can hold 2 Large, 4 Medium, 8 Small, 32 Tiny, 128 Diminutive, or 512 Fine or smaller opponents.

Amphibious (Ex): Although kuo-toa leviathans breathe by means of gills, they can survive indefinitely on land.

Immunities (Ex): A kuo-toa leviathan is immune to poison and paralysis. The various hold spells also have no effect on it, and its keen sight automatically detects figments for what they are.

Improved Evasion (Ex): When subjected to an attack that normally allows a Reflex saving throw for half damage, a kuo-toa leviathan takes no damage if it makes a successful saving throw and half damage even if the saving throw fails.

Keen Sight (Ex): A kuo-toa leviathan has excellent vision thanks to its two independently focusing eyes. Its eyesight is so keen that it can spot a moving object or creature even if it is invisible or ethereal. Only by remaining perfectly still can such objects of creatures avoid the leviathan's notice.

Light Blindness (Ex): Abrupt exposure to bright light (such as sunlight or a daylight spell) blinds a kuo-toa leviathan for 1 round. In addition, the creature is dazzled while operating in bright light.

Sea Mother Blessing (Ex): A kuo-toa leviathan has a sixth sense that allows it to avoid blows. The creature gains a bonus to its Armor Class equal to its Wisdom bonus. This bonus applies even against touch attacks or when the leviathan is flatfooted. It loses this bonus only when it is immobilized or helpless.

Slippery (Ex): A kuo-toa leviathan secretes an oily film that makes it difficult to grapple or snare. Webs, magic or otherwise, don't affect the kuo-toa leviathan, and it can usually wriggle free from most other forms of confinement.

Uncanny Dodge (Ex): A kuo-toa leviathan retains its Dexterity bonus to AC, even when caught flat-footed or struck by an invisible attacker.

Skills:A kuo-toa leviathan receives a +8 racial bonus on Escape Artist checks and a +4 racial bonus on Spot and Search Checks. It has a +8 racial bonus on any Swim check made to perform some special action or avoid a hazard. A kuo-toa leviathan can always choose to take 10 on a Swim check, even if distracted or endangered. It can use the run action while swimming, provided that it swims in a straight line.

Kuo-toa leviathans usually stay in deep water, where they're most maneuverable. Their forays onto land are rare and prompted only by the most extreme of situations. Wherever they fight, kuo-toa leviathans use their enclave of regular kuo-toas, monitors, and whips to swarm intruders. While the opponents are fighting off the horde of kuo-toas, the leviathan is free to focus on dispatching one foe at a time.

Aquatic Subtype

Creatures with the aquatic subtype always have swim speeds and thus can move in water without making Swim checks. An aquatic creature can breathe underwater. It cannot also breathe air unless it has the amphibious special quality.