Ok, I'm pretty certain this thing will need some work. However, its about as close as I could get to a living, sentient, free-flowing body of water.
Colossal Outsider (Extraplanar, Water)
Hit Dice: 31d10+124 (294 hp)
Speed: 90 ft., swim 90 ft.
Armor Class: 17 (+6 dex, +9 natural, -8 size), touch 8, flat-footed 11
Base Attack/Grapple: +31/+55
Attack: Rushing Waters
Full attack: Rushing Waters
Space/Reach: 80 ft./60 ft.
Special Attacks: Drink not the water, misleading mists, rushing waters, spell-like abilities
Special Qualities: Blindsight 60 ft., damage reduction 10/-, darkvision 60 ft., elemental traits, flowing form, immunity to acid, electricity, and weapon damage, truley hidden, vulnerability to dessication
Saves: Fort +23, Ref +25, Will +20
Abilities: Str 26, Dex 22, Con 19, Int 10, Wis 16, Cha 11
Skills: Balance +40, Climb +42, Jump +42, Knowledge (nature) +34, Knowledge (the planes) +34, Listen +39, Spot +39, Swim +42
Feats: Ability Focus (misleading mists), Alertness, Combat Expertise, Diehard, Dodge, Great Fortitude, Improved Initiative, Improved Trip, Lightening Reflexes, Mobility, Run
Environment: Plane of Faerie or any forest or mountains
Challenge Rating: 20
Alignment: Usually neutral
Advancement: 32-60 HD (Colossal)
Level Adjustment: -
Hidden streams are wonderous beings that flow through the deadly forests of Faerie. To fey and other beings of Faerie they are normally harmless, but they are a great danger to others. Though generally found only on their home plane they also visit the material plane, sometimes for centuries, but only within the deepest forests or most distant mountains.
They are willing members of the Unseelie Court however, and from time to time are sent to collect mortals. They then wait within the wilds near civilization for woodsman or mountaineers who are surprised to find a stream where none were to be found before.
Pure waters composed of morning dew and the perfumed rains of Faerie mixed in with the intense underlying magics of the plane, hidden streams appear as anything from a narrow creek to a gushing brook, though whererever they flow the greenery about them flourishes. They are particularly fond of steep, rocky forests where they may cascade down over and over as they loop in on themselves.
Hidden streams understands Fey, but can only speak it to those immersed in its substance.
Hidden Streams on the prowl stealthily follow a potential victim, shooting ahead to scout before coming back, repeating until the victim nears an ideal site. It then begins to produce its mists and allows itself to be heard and seen. Forming into crystal pools, and growing thick carpets of mosses and ferns about itself, it lulls the victim into complacency. If the victim drinks of it, or attempts to wade it, it springs into action.
Crooked Path (Ex):
When a hidden stream uses the run action it may make three turns of up to 90 degrees during its movement.
Drink Not the Water (Sp)
Though it appears as water, and tastes and smells like water, a hidden stream is not, strictly speaking, water. Water breathing creatures within its substance do not gain any benefit from its inhalation and can drown as easily as an air breather. Those who drink of a hidden stream's substance, or begin to drown within it, must make DC 29 will save or be affected as by the Dominate spell for 1d6 hours. Those who make the save are still nauseated for 1 round. The save DC is fortitude based.
Fey who drinks of its substance however are not affected, finding it immensely refreshing. As well, aquatic fey may breathe while submerged within it.
Flowing Form (Ex):
A hidden stream has the traits of an elemental. As well, it may occupy the same space as creatures up to one size category smaller than itself. A hidden stream cannot be bound, bull-rushed, grappled, or tripped.
Misleading Mists (Su):
A hidden stream can cause a dense mist to form over the course of 2d6 rounds at will. This functions as Fog Cloud, except that it extends forty feet in all directions of the stream. Those within must make a DC 27 Will save to keep from losing their sense of direction every minute they remain within the fog. If the saved is failed they meander blindly through the mists, though must remain within its boundaries unless personally directed or pulled out by one not affected. This is a mind-affecting effect. Natural or supernatural senses and special abilities that allows one to intuit direction are overpowered by these mists unless the victim possesses higher hit dice than the stream. The save DC is Charisma based.
Rushing Waters (Ex):
Though a hidden stream is normally very gentle to begin with, it can suddenly increase its velocity. Those who occupy the same space as the stream at the end of its round must make an opposed strength check as a Trip attack (the hidden stream gains a +4 bonus to its check thanks to its Improved Trip feat). If the victim looses the check he is swept off his feet and take 1d6 points of nonlethal damage a round. Even if they make the save they must succeed on an additional DC 20 Reflex check if they wish to move that round. If they fail they are likewise carried off. Even if the victim remains unmoving, they must make another check each round. Those who are swept away risk going under unless they succeed on a DC 20 swim every round.
Those who drown within a hidden stream do not die, but instead going into a state of suspended animation. Once the stream has collected as many as it wants it uses its planeshift ability to return to the plane of faerie with its new catch.
At will/Find the Path, Plant Growth. 1/day Planeshift. Caster level 30.
Truley Hidden (Su):
A hidden stream cannot be seen if it so wills, normally remaining invisible as the spell until it uses its misleading mists ability. It then allows itself to be seen, hoping its mislead victim will try to quench its thirst with its substance.
Nor can it be heard unless it wishes to be, able to control the volume of its passage. It often remains in place and gradually increases its noise as it or its opponent draws near. At all times its substance is easily felt however, feeling as cold, running water.
Vulnerability to Dessication:
A hidden stream takes half again as much (+50%) damage as normal from dessication spells and effects, regardless of whether a saving throw is allowed, or if the save is a success or failure.
A hidden stream can always choose to take 10 on a Swim check, even if distracted or endangered. It can use the run action while swimming, though need not swim in a straight line.