In Brian Froud’s Faeries, the nuckelavee was depicted just as a fist interjecting itself onto the page to crush a pixie. In “basic” D&D/the Known World/Mystara, it was a translucent, yellow-veined, black-blooded amphibious centaur with fear, blight, and cold powers—a description that seems to get closest to the original Orcadian (there’s a word I don’t get to use every day) folklore. In the 3.5 edition of the world’s oldest role-playing game, it only appeared on the Web (as the nuchlavis, courtesy of Robert Weise), where it first gained its proper fey status.
The Pathfinder version diverges a bit from the original mythic material, but I’ll give it a pass as the changes probably make it more useful in the average campaign—taking it out of the bogs and making it a red-blooded spirit of corruption and pollution. (At least it keeps the perfectly named nuckelavee disease: mortasheen). The nuckelavee is like an allergy, or the fever that fights the pathogen so hard it kills the host—nature’s wrath boiling over to scald not just the polluting offenders, but mankind in general. If you want a fey that serves as the manifestation of a xenophobic bog’s rage at being disturbed, you have it. If you want a fey that is the perrenial plague of an isolated seaside town, you have it. And if you want the river to cry out against pollution in the heart of an urban campaign, the nuckelavee works for that, too.
And if you want a truly nasty semi-original fey to throw at your players (especially as we move into the Reign of Winter Adventure Path) the nuckelavee is just begging to be beefed up: Turn it white; add a few more Hit Dice, cold powers, and blight effects (enough to get it to CR12, methinks); trade the longsword for a wicked claymore or flail; and you’ll be good to go.
One more note: the nuckelavee supposedly hates the Mither o’ the Sea, a benevolent summer water spirit. In your campaign this might be a unique nixie, nereid, triton, marid, storm giant, or even a brine dragon.
Three summers in a row, the stench from the river Gough has brought the city to a halt and driven Parliament to summer a day’s journey upriver. During this third summer a nuckelavee rises from the effluvium and begins slaughtering anyone who comes near the riverbanks—even the very crews meant to clean the filth.
When the red tide comes to Nell, the people cower in their houses for fear the nuckelavee will come for them. The wise women caution against burning the red seaweed, saying the practice enrages the nuckelavee. The lairds retort that the fey never attacks the rest of the year no matter how much seaweed is burned, and that the potash and soda produced keeps the town coffers full.
Someone is murdering the alchemists of San Xephar, a sun-baked city-state of humans, ifrits, and reptilian races. A gnome druid from Westlund far to the north claims to know the cause of the bloodshed, but vishkanya thugs—kin to the very alchemists being targeted—seem determined to silence him before he speaks truth to power.
—Pathfinder Bestiary 3 203
I love the nuckelavee. I would have killed to write an “Ecology” article for one, back when Dragon was a thing. And do follow the Orkney links above—they’re a treasure trove.