Aboleth (CR 7)

Huge Aberration (Aquatic)
Alignment: Usually lawful evil
Initiative: +1 (Dex); Senses: darkvision 60 ft., Listen +16, and Spot +16
Languages: Aboleth, Undercommon, Aquan

AC: 16 (-2 size, +1 Dex, +7 natural), touch 9, flat-footed 15
Hit Dice: 8d8+40 (76 hp)
Fort +7, Ref +3, Will +11
Speed: 10 ft., swim 60 ft.
Space: 15 ft./10 ft.
Base Attack +6; Grapple +22
Attack: Tentacle +12 melee
Full Attack: 4 tentacles +12 melee
Damage: Tentacle 1d6+9 slime
Special Attacks/Actions: Enslave, psionics, slime
Abilities: Str 26, Dex 12, Con 20, Int 15, Wis 17, Cha 17
Special Qualities: Aquatic subtype, mucus cloud
Feats: Alertness; Combat Casting; Iron Will
Skills: Concentration +16, Knowledge (any one) +13, Listen +16, Spot +16, and Swim +8
Advancement: 9-16 HD (Huge); 17-24 HD (Gargantuan)
Climate/Terrain: Any underground
Organization: Solitary, brood (2-4), or slaver brood (1d3+1 plus 7-12 skum)
Treasure/Possessions: Double standard

Source: Monster Manual

Transformation (Ex): A blow from an aboleth's tentacle can cause a terrible transformation. Affected creatures must succeed at a Fortitude save (DC 19) or begin to transform over the next 1d4+1 minutes, the skin gradually becoming a clear, slimy membrane. A transformed creature must remain moistened with cool, fresh water or suffer 1d12 points of damage every 10 minutes.

A remove disease spell cast before the transformation is complete will restore an afflicted creature to normal. Afterward, however, only a heal or mass heal spell can reverse the change.

Psionics (Sp): At will - hypnotic pattern, illusory wall, mirage arcana, persistent image, programmed image, project image, and veil. These effects are as the spells cast by a 16th-level sorcerer (save DCs, where applicable, are 13 + spell level).

Enslave (Su): Three times a day, an aboleth can attempt to enslave any one living creature within 30 feet. The target must succeed at a Will save (DC 17) or be affected as though by a dominate person spell cast by a 16th-level sorcerer. An enslaved creature obeys the aboleth's telepathic commands (but will not fight on its behalf) until freed by remove curse or dispel magic, and can attempt a new Will save every 24 hours to break free. The control is also broken if the aboleth dies or travels more than one mile from its slave.

Mucus Cloud (Ex): An aboleth underwater surrounds itself with viscous cloud of mucus roughly 1 foot thick. Any creature coming into contact with inhaling this substance must succeed at a Fortitude save (DC 19) or lose the ability to breathe air for the next 3 hours. The affected creature suffocates in 2d6 minutes if removed from the water. Renewed contact with the mucus cloud and failing another Fortitude save continues the effect for another 3 hours.

An aboleth attacks by flailing with its long, slimy tentacles, though it prefers to fight from a distance using its illusion powers.

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.