Suisseen

On the Elemental Plane of Water, a suisseen is a thin clear skin swollen with liquid. Where it ends and the sea begins is unclear: the membrane is plainly the creature, but part of the surrounding water and much of the water within seem to belong to it too. Flute-shaped tubes pierce the skin, and by pumping water through them it swims, feeds, digests, excretes and even talks. That pumped water is part of its body, not just its environment.

The caustic membrane burns anything it touches or strikes for 1d4+1 points of damage, but the creature's main weapon is its watery bulk, which slams foes like an elemental's wave for 2d8+2.

Away from its home plane, it can also try to drown air-breathers. This needs its own attack roll and can accompany a slam. A struck victim makes a Constitution check; on a failure he is helpless and takes 2d6+2 as water floods his throat, and he checks again every round. His first success means he spends that round pulling free and stops checking unless another drowning attack lands.

Attackers must choose a target. The surrounding water is AC 0 and harmed only by +1 or better weapons. The membrane is AC 3 and vulnerable to normal weapons, but reaching it means stepping inside the watery envelope, which grants the suisseen a free strike on that foe even after it has attacked that round. Fire does nothing to it; lightning and cold deal double damage.

Untainted suisseen gather in small bands in the elemental sea; how they breed or organize remains a mystery.

A cult called the Tidebound, made up of humans, sahuagin and mermen, now worships the suisseen themselves, calling them 'the door and the way,' though it once saw them only as a bridge to true water elementals. Its evil members believe water multiplies life force and offer living victims for power and long life. Contrary to rumor, they know the creatures are many, not a single god. Because the cult works mostly on the Prime Material Plane, most suisseen have no idea they are worshiped. Cultists call one with a special summoning spell and feed it victims. The rite likely grants nothing, but suisseen summoned repeatedly come to crave it, slide from neutral toward neutral evil, then start hunting humans and other thinking prey.

Such creatures try to strike deals to stay on the Prime and keep eating, but without magic no one can understand their gurgling, piping speech. Back home they prey on merfolk, tritons and sahuagin and attack their own kind as rivals.

Sages doubt the theory that they are magical blends of water elementals and something like a gelatinous cube. They seem closer kin to the water grue, a territorial elemental scavenger of the same plane.