Part 1
They would pop out from beyond the current world borders(HERE BE DRAGONS), on knarrs(1-3 knarrs, 3-6 men in each).
They would raid & plunder all villages remotely near them and all/some small hermits near their aggro range(sad but such is life).
Their aggro range would be larger than animals, they would have some method to harass paliwalled places but would not attack brickwalled places, because they know their own strength.
They'd pillage, plunder, kill and burn, until their knarr is full enough, then return beyond the world border to take back the loot.
They could be attacked without rage(counted as animals) and would as such need some defining characteristics.
Don't know how strong to make them though...
Part 2
If and when world borders are expanded, they'll still appear from even further, but they'd have their own villages on the map(for us to get revenge).
The villages have mostly women, but some warriors too(no children cause why??).
The village would have an actual loot house, which would contain some random loot from all the pillaged stuff(stuff taken beyond the prev world border).
Stuff could also be brought, by vikings, to villages by then.
Even though they did do daily work, have villager NPCs in village houses until under siege.
Under siege, men(the few left behind) & women(weaker than men) would defend the village, assumed children will stay safe indoors, dead when raid successful)
Village fighting power dependant on size, needing a decent player force to raid.
This would give mayor powers something to deal with, while hermits would be still raided, but not because of how players are, but because how the game is

No idea what you'd get from a viking corpse, what equips they would wear(unique clothing? gildable ofcourse, maybe with gilds already) that you could steal, and what the skull would be named as.