The Murdhuacha and the Undersea

The many bloodlines of the Kithain aren't limited to life on land. Underneath the sea live Seelie and Unseelie fae creatures who have also undergone the Changeling Way. They have had little contact with their cousins on land since the Sundering, however, shunning the "dirtwalkers"; even their limited knowledge of the Changeling Way came via the selkies. Recently, however, the deepening encroachments of humanity -- and thus Banality, which they call "the Coldness" -- have made it necessary for these arrogant fae (who in their opinion do rule most of the world, after all) to swallow their pride and seek aid from the changelings who make their homes in the "Not-Sea."

The merfolk and mudhuacha are each linked to a marine animal type which their lower body reflects, usually being some sort of aquatic tail, but this splits into legs when they venture onto land. They gain some advantages of the creature they are tied with, usually have gills (though some tied to marine mammals or amphibians have to surface for air periodically), and all possess a peculiar alien beauty. Banality is very fearsome for them (indeed, losing one's fae nature at the bottom of the ocean is fatal), and certain skills are unavailable to them due to their being cut off from mortal society.

The Murdhuacha, or "Merrow," are linked to invertabrates: squids, urchins, jellyfish, etc. They are cold, calculating and inhuman. They are "the first dreamed of Dagon." These sinister Thallain are from where most of man's sea horror stories originate. These Thallain are even more alien and inhuman than their mer cousins. Gigeresque nightmares spawned of invertebrate sea creatures, they live in their aquatic hives -- often sunken ships and other wreckage, for which they are themselves not infrequently responsible -- and pursue their own sinister agenda. Like the merfolk, the merrow do not breed with humans, but do frequently use human corpses as depositories for their eggs which the females lay hundreds at a time; these, against the received wisdom, hatch into what appear to unenchanted observers to be human babies, but to fae sight are indistinguishable from mer nereids. Very few of these survive to undergo the Rite of Dagon, in which the young nereid receives his Apsara and becomes a nix. Female merrow are often called "sirens" and the hideous males "nucks."
The Merrow enjoy plucking mortals from the coast and drowning them in the ocean to feast on their corpses or worse yet mate with them, eviscerate them, and place their eggs in them for incubation. They are cunning and intelligent without any human morality. While animalistic and instictual, they are not stupid, in fact they are far from it.

While the females can have an entrancing if disturbing beauty, the males are absolutely hideous. Male murdhuacha have jellyfish-like flesh that make them appear skinless horrors from the waist up. Both sexes are an unsettling mix of humanoid and cephalopod, jellyfish, annelid, or other such creature. Although they can be truly alien in appearance, the female's looks can stun you cold.

Nereid is the first and youngest seeming, wherein the Murdhuacha are no different from their beautiful cousins the Merfolk. Nereids are indistinguishable between these two kiths until they undergo the Rite of Dagon, wherein the Nereids mature into having an aspera or animal half. In the Nereid state the Mer and Merrow are both grey-blue and have fish eyes and lack gills and must survive and breath air through their mother's milk. Nereids take one year to mature before the rite of Dagon and are babies in their Seeming - if somehow they should be forced into it.

The Rite of Dagon is an ancient ritual, wherein murdhuacha parents take their young nereids to a Grotto, a sacred and mystical place of power (not unlike a Freehold). The Baby, the Nereid, waits in the Grotto and the next animal that appears bonds with the Nereid and transforms him into a Nix. If the animal is an invertabrate the Nereid will be born as a Merrow, if not, they will be born as one of the beautiful and sidhe-like Merfolk. The next stage in life is the Nix, wherein they have undergone the Rite of Dagon changing them into Murdhuacha or the Rite of Vatea transforming them into Merfolk. The Nix stage compasses both childhood and adulhood, much as the wilder years amongst landbound kithain. The last stage is Naugs wherein they mature, retire, and serve as advisors and teachers.
When the merfolk lose their fae nature, banality kills them by drowning or crushing them under the ocean's pressure. The tales of their Sundering make one think that the land fae have it easy. When Murdhuacha are stripped of their Glamour, or when they fall into the Forgetting, or even hide within their Seeming or lie Chimerically slain, if they are beneath the waves they will drown just as would any human.