A couple of days ago, Blue Maru asked whether there would be monsters and magic in my viking campaign
Yes, there will be both magic and monsters the viking age, but neither will be as prevalent as in a normal D&D campaign. One of the reason I chose to work with Chainmail as the bases for my combat system is to de-D&Dize the mechanics and, hopefully, some of the underlying assumptions of the game. But Anyway, Blue Maru, you asked about monsters and maqgic. Well, I'll cover monsters today and magic in a day or two.
I have plans for a number of monsters appearing in the campaign. The most prominent of those are elves, dwarves, giants, ghosts, faeries, and, of course, dragons. Note that I have added my own additions and subtraction to these creatures, and these representations are not mythologically accurate accounts of Norse mythology.
Elves: The spirits men who have made exceptionally great or terrible achievements in life may rematerialize as elves after their death and endowed magical powers, which they use both for the benefit and the injury of men. There are two types of elves, light elves, who where heroes of men and live in Álfheim, and dark elves, who were the bane of men and reside in the cavernous realm of Svartalfheim, deep under the earth--fearful of the sun's rays. Elves retain their past personalities and skills as they pass
Dwarves: Dwarves are skilled craftsmen, and most of their magic involves labour, craftsmanship, and metallurgy. In their underground mountain hall, Nidavellir, dwarves make the treasures of the gods (such as Thor's hammer) and hold a repository of secret wisdom. They live under the earth, away from light, because sunlight causes them to turn to stone. As a people, dwarves are stubborn and easily offended.
Giants: Giants are characterized by their hideous size, superhuman strength, and are often act in opposition to the gods. Although most gaints appear as oversized men, some giants sport claws, fangs, and deformed features, such as having two heads. Unlike the average D&D giant, some are extremely wise and knowledgeable; others are dumber than rocks.
Ghosts: Ghosts of dead people. Some dead people, not content to lie in their grave mounds, live on after death. Usually, these are people who committed an evil deed during their life. The ghosts return to harass the living, causing illness, insanity, and death. The only way to force a ghost to move on to the afterlife, is to fulfill the dreams and desires that they had in life or to strike them with a magical sword.
Faeries: Spirits of nature, faeries reside outside the realm of good and evil; they are tricksters of the purist sort, playing games with mortals that pass into their dominion. Faeries are sometimes attached to particular families or natural landmarks and will use their menacing games to harass the families enemies or intruders of the land.
Dragons: These great, winged lizards make their homes in cavernous lairs where they amass knowledge and treasure stolen from men, elves, dwarves, and giants. It is said that the dragon's fiery breath can slay the mightiest of giants and melt the strongest metals.