"There was a disturbance 6 feet out, into the sea. From a bubbling froth rose a long skull, and behind it, a second one, human, topped by thin, wiry hair. A deep gurgle came from the conjoined corpses. "

The Nuckelavee is an Orcadian demon that is part horse, part man, and entirely evil. Its body is devoid of skin, comprised instead of rotting, festering muscle, sinew, and aged bone; from its gaping, skeletal maw, it spews a deadly, horrendous vapor that wilts crops, kills livestock, spreads plagues, and causes droughts. Accustomed to the ocean, Nuckelavee cannot tolerate the touch of freshwater. When it rains, the sea devil is forced back into the salty depths of Orkney, where it slowly recovers. While underwater, its form changes.

While the Mither o' the Sea confines the Nuckelavee during the summer, it can break free if enraged—and nothing angers the demon more than the smell of burning seaweed. Coming to shore, rampages caused by these sorts of affronts last longer and are more violent; much of the demon's hatred is directed to horses.