Kelpie

A kelpie is clever, animate seaweed that exists to drown fools. Its natural form is a tangled mass of weed, but it can mold itself into a green-clad woman, a green horse or a hippocampus, though it is always made of seaweed whatever the shape. It speaks telepathically to anyone it holds.

The kelpie has no real attacks of its own. When a humanoid male draws near, it takes the shape of a woman or a mount, a crude imitation that is 95% likely to be seen through in daylight, so it relies on magic. Once a day it can cast charm, with -2 to the victim's saving throw, and it works only on humanoid males. A man who fails sees it as the woman or steed of his dreams and plunges into the water to seize it. The kelpie coils around him, and he drowns contentedly, taking 2d10 damage each round until he dies, reaches air or is protected from drowning. Even if it cannot reach him, the charm drives him to swim out and drown himself. The kelpie then hauls the body to its lair to eat.

Men who can breathe water survive but still blissfully wrap themselves in its fronds; the baffled kelpie makes no attempt to remove their protection and may even enjoy their continued company. On land, a charmed man defends it furiously against his own friends, enraged by their apparent betrayal. The charm ends at once when the kelpie dies, or when it simply expires.

Its soaking body takes half damage from fire, or none with a successful save, and it cannot be seen with infravision.

One tale says a vengeful sea god made kelpies to punish men who sailed without paying him his due; since few sailors then were women, women were spared. A rarer version credits Thessaryne, princess of evil water creatures, who made all women immune out of regard for her own sex.

Kelpies usually live in the upper 100 feet of the ocean but adapt to any water, even underground or artificial pools. In human or horse shape they can walk on land for 1-3 hours and may still charm men there, who, safe from drowning for now, follow, carry and guard it.

They reproduce asexually, growing to 7 feet and then splitting into 2-4 new kelpies, as often as monthly when victims are plentiful and local fish have not grazed on them too heavily. They have no defense against fish and other grazers, which treat them as ordinary seaweed. Any treasure lies among victims' remains, often in decaying corpses still caught in the fronds.

Kelpie sprouts fetch 500 gp. Too small for magic at first, a sprout grows into an adult in a month with proper care. Some kelpies serve mighty allies or watch over drowned treasure.