So what is the Nuckelavee?
The Nuckelavee is a monstrous type of fairy found in the folklore of Scotland and the Orkney Islands. It is as far removed from our modern conception of a fairy as possible, and its name is still considered unlucky in some parts of the Scottish Isles.
A horrific demon of the seas, Nuckelavee was a godlike horror that was intangible and invisible while in the sea, taking corporeal form only once it reached land. The form it took was akin to a living nightmare and was described as either a horse, a man, or a fusion of both. In both forms the fairy was skinless, with pulsating veins and muscles exposed.
Nuckelavee was gigantic in size, yet its features were grotesquely mismatched with a head so horribly large, it would roll to one side. Its arms were so long they'd drag along the ground. In addition, Nuckelavee's hands were monstrously huge: often it would have only a single Cyclops-like eye on its head. The horse's legs were said to be akin to flippers.
Further horror came from Nuckelavee's gaping mouth, which was filled with rotting fangs and a supernaturally putrid breath that could strike living beings dead or infect them with disease. The creature spread all manner of catastrophe to mankind and was said to be responsible for plague, famine, drought and more. According to legend the Nuckelavee's power was only rivaled by its undying hatred of humanity.
The burning and harvesting of seaweed was said to invoke the Nuckelavee's wrath even more than usual. In retribution the evil fairy would inflict the islands' horses with a deadly disease known as mortasheen.
Only two possible ways existed to stop the Nuckelavee: the fairy's inability to cross freshwater and the intervention of a benevolent spirit known as the "Sea Mother" who held the power to undo Nuckelavee's curses and drive the demon back to the depths (she was presumably a powerful fairy or angel, perhaps even an aspect of some ancient goddess).