Rattskjarl the Frost Wolf
by HåvardStats: As Howling Demon (or Frost Wolf, see below)
HD: 16
Hit Points: 92
AL: N
Rattskjarl was originally Wolf Demon of Helheim. At one point he made an agreement with a human warrior from Ostland on Mystara. The demon would serve the human, named Ulf the Mad, untill the warrior died. The demon took the form of a Frost Wolf (see AC9) and pretended to be the warror's animal companion. At the point of the warrior's death, Rattskjarl was to bring Ulf's soul to the demon's master, Helreginn. However, when the time came, Ulf bargained for his life. If the warrior could best Helreginn, he would keep his soul and be returned to Mystara. In addition, he demanded that Rattskjarl would be bound to the service of him and his descendants as long as his bloodline existed. Ulf then bested Helreginn (see details above).
Helreginn's plan was to use Rattskjarl to gain new souls of Northmen, even though Ulf escaped him. The Wolf Demon must return to Helheim once every year and report before once again reuniting with Ulf's clan. However, Rattskjarl is actually fairly happy with this arrangement as he has gotten quite fond of the humans of Ulf's Clan and does his best to protect them. He will change his reports to prevent any of those souls to be brought to Helreginn.
While on Mystara, Rattskjarl cannot use any of his demonic abilities. Use stats for Frost Wolf, but retain his original Hit Points. He may at any point spend an action to return to Helheim carrying a single mount.
Behind the scenes: Rattskjarl was created for the Mystara chapter of the book Dungeons & Dragons: Worlds and Realms by Adam Lee. I changed the creature considerably, as explained in my writeup of Ulf the Mad.
Note: I have updated the details on Helreginn to reflect these new elements.