NORTHERN FEY
Huldr:
The name Huldr is sometimes used as a general term for Fey, but also just as often refers to a female creature similar in many ways to Dryads. Like Dryads, they appear as beautiful human women. I would probably suggest a nordic appearance for the Huldr. While appearing to be fragile women, they have supernatural strength. Perceptive adventurers may also notice the cow or fox-like tail of the Huldr, though she always tries to keep it hidden under her long skirt/dress. Like the Dryad the Huldr charms her victims and the Huldr then takes the helpless male with her into the earth. She can pass through mountain rock like a Dryad passes into her tree. While tales of the Huldr women are most common, males also exist. The Huldr live in great kingdoms beneath under ground, often quite close to human settlements which sometimes leads to problems between the two races.
Nøkk (Nix)
The Nøkk is a male water spirit living in lakes and streams, probably related to Nixies. He is often depicted as a male human with glowing eyes and hair overgrown with water plants. The Nøkk is known for luring women and children into drowning. His favorite way of doing this is to take the shape of a beautiful stallion and trick victims to attempt to mount him. Once astride the horse, the stallion will set off in a wild raze eventually leaping into the water pulling his victim with him. According to some myths, pouring a bit of alchohol or tobacco into the lake will please the Nøkk and he will teach you some enchantments. The Nøkk is vulnerable to daylight and does not like to leave the water for extended periods.
Fossegrim
The Fossegrim is another water spirit related to the Nøkk. These appear as attractive (but usually bearded) men living in water falls. They are known for their skill with playing the fiddle (or even Norwegian Hardinger, a special fiddle used for the Rohan theme in the LotR movies among other things). They are enemies of the Nøkk who will try to destroy the Fossegrim's musical instrument should they meet. Fossegrims may be befriended by offering them lamb meat. If their trust is betrayed however, the Fossegrim will dance naked in front of you which is a traumatic sight (I propose turning victims to stone or similar effect). Befriended Fossegrims may teach their amazing musical skills to visitors.
Kvernknurr ("Mill Mouths")
Yet another water dwelling Fey, the Kvernknurr live in streams or rivers and cause trouble for millers as they are able to stop a watermill by chewing onto the water wheel with their gigantic mouths. I don't really know what they look like except for having mouths the size of a doorway. I imagine them as goblin-like with pointy ears and small hairy bodies.
Draug (Draugr, pl: draugar)
These aren't really Fey, but material Undead similar to zombies or Ghouls. They usually drowned sailors rising from the dead to pull others into their watery graves. The draugar appear as bloated corpses whose skin has begun to turn blue. Sometimes they swim and try to turn boats around. At other times, they sail ghostly small boats (or even ships) with torn sails. They are said to be a warning of death. Those who see a Draugr will not reach land alive (this is a curse, and a curse should be allowed). Draugr has to stay near the sea, though they often hide on the beach, where they often take the shape of large rocks. They fear clergy and are quite likely susceptible to turning. Older myths distinguish between the Sea Draugr and the Land Draugr. Most of the above refers to the former.
Marmæl
These tiny Fey appear like small Merrow and live on the bottom of the sea. They are mostly friendly and hate the Draugar and are able to repell them. If approached in a friendly manner and if they are offered clothes (they may ask for it), they will repay this generousity by answering questions. They possess great knowledge. However, they are proud and hate stupid questions. If annoyed in such a way, they may use their weather controlling powers to create violent storms. While they live under water, they have no problems breathing above water.
PS: In addition to my own understanding of these things, I consulted the wikipedia and the Norwegian RPG draug which is quite true to Norwegian myths.
Any ideas for how these could be used in Mystara?
Havard
My initial thoughts are that, since these are associated with Norse myth and flavour and are best used in that context, you could say that these are (or were) fey that were exposed to the blast effects from the GRoF.
It is said that the radiation from it was responsible for creating the monsters of Mystara and could therefore be the reason we have these specific fey in the north lands (close to the Blackmoor disaster) and not found everywhere.