Nereid

Encyclopedia Entry:
Aquatic monsters known as “daughters of the sea.”   They’re a kind of sea-loving succubus that inhabits the ocean. Women who drown in the ocean and have their bodies completely violated by the magical power of the sea god “Poseidon” are changed into nereids. Normally they are calm like the sea, and free-spirited, but when they have sex with a man they go wild like a stormy sea and indulge in the pleasure.

When human women are changed into “nereids,” from having their whole body violated by Poseidon’s magical power, they are overcome with waves of strong pleasure and sexual impulses. Due to this, the restraint and repression they had as humans was completely swept away. They get a sense of freedom that they never experienced as humans, and become filled with happiness. And then their body and heart become mamono, and they start thinking of life in the sea as the ultimate happiness. If a woman who was changed into a nereid had a man she loved on dry land, she will change herself into a human form, and go onto land to seek him out. Then, using her mamono transformed body, she will seduce the man, and sink him into the depths of pleasure, as if he were sinking to the bottom of the sea. After capturing the man in this way, the two of them will return to the sea.

Nereids who bring back men fundamentally just keep having sex at the bottom of the sea, but unmarried nereids who haven’t gotten a man come up to the surface of the water, so they are very dangerous beings. Besides trying to drag men they like into the sea, they do the same to women in an attempt to change them into nereids in order to share the feelings of liberation and happiness that they have obtained.