There are hearthlings who were wicked during their lives, did not get a proper burial or for another reason, walk after death.
They rise coming the stillness of winter, look enviously upon living hearthling settlements to amass at walls, bashing openings inside, devouring cattle, destroying hearthfires and stealing treasure to their burial mound.
The more populated a land is the greater the danger is posed by the again walkers to individual hearthlings, as they are stronger and more numerous near prosperous kingdoms.
It is known that before a group appears, you can smell a stench of decay in the air. Great city dwellers would also often spot flocks of carrion bird high and away in the air above a horde as a portent of doom.
The draugr feel no pain, and as such, are hard to butcher with conventional weapons. A shivering peasant may pray for his ancestors, a community priest or wandering druid may bless weapons, a dutiful ranger prepares his arrows with tar during autumn, a wealthy king orders the forging of special silver weapons for his knights.
Those who are not ready for the coming of a wicked winter will look to other hearthlings for help, abandon the land they live upon, contend with the dead by dying or, secretly do sacrifices to delay their doom, sacrifices which raise the undead horde's strength for the next winter.
Not even islanders are safe as the undead come from the sea, it is even said some have claimed ships and pirate the vast frozen oceans for merchants.
The shorter hearthlings who live far beneath the earth are said to be safe, but it is common for settlements under the stone to go quiet during some winters, sometimes forever...