The Viking King, Harald Sigurdsson (called Hardrada), King of Norway, was for a time in the service of the Byzantine Emperor in Constantinople.
Strange but true. Harald Hardrada (hard ruler), the Viking King of Norway spent a number of years in service to the Byzantine Emperor in the Varagian Guard. Harold was not always the king of Norway.
Harald's brother Olaf Sigurdsson had tried to take the throne of Norway from Cnut the Great (King of England, Norway and Denmark). Olaf failed in his first attempt. The second attempt was with Harald at his side (Olaf's half brother), and they lost the battle of Stiklestad in 1030 AD. Harald fled to the east and exile. Olaf was killed in this battle.
When Harald returned, he successfully conquered Norway as king. He then made a bid against the Anglo Saxon King Harold Godwinson, and died at the Battle of Stamford Bridge.