hi! there's lots of room for more input, but meanwhile, you may be interested in these posts
p.s. The word "sellsword" comes from fantasy novels; the term you're after is "mercenary"
They were originally Viking mercenaries from the Volga river area, although later they accepted a large number of Anglo-Saxons who had fled England after the Normal conquest. Being foreigners meant than, like the Janissaries in Turkey, the Mamelukes in Egypt, the Swiss Guards, and the French Foreign Legion, they owed no regional or political allegiance to any of the squabbling factions competing for power within the empire.
The Volga (Russian) Vikings came originally from Sweden, and established trade in slaves, furs, etc. with the Baghdad Caliphate. They had trading posts (later cities) all the way north-to-south from the Baltic to the Caspian Sea, and their excess manpower was recruited by the Byzantines as mercenaries. After that, recruiters all over the Norse world channeled volunteers into Byzantine service. A cathedral in Istanbul has Norse runic graffiti scratched into a railing.
They were large by Asia-minor standards, and fought in nearly impenetrable shield walls that made them less vulnerable to archers than other forces. They were an immovable anchor around which an army made up of mostly peasant levies could be formed.
The Anglo-Saxons got there after fleeing the Normans via (I think) Denmark and Saxony. Ironically, the Varangian Guard was eventually defeated by the Normans who took over Sicily. When the Crusaders attacked Constantinople in 1203, the Varagians were abandoned to die in place by their emperor, and they cut a deal to escape. After that, the Byzantine royal guard was made up of men from Crete.
Like the Praetorians, Mamelukes, etc. the Varangians eventually let their power go to their heads, and meddled in politics to the detriment of national security. See "Lost to the West" for a thorough history of the Byzantines.