Harald Hardrada was king of Norway. He learned his trade as captain of the Byzantine Emperor’s Varangian Guard. There, Hardrada amassed huge wealth and a reputation for violence. He returned home to Scandanavia, captured and consolidated the Norwegian kingdom, and pursued a claim against Denmark.
He also invaded northern England in 1066 with a fleet of around 300 ships!
Harald Hardrada beat the Anglo-Saxon forces of Mercia and Northumbria, but died in battle against King Harold of England (a battle the Norwegian invaders lost). King Harold’s tired forces were force-marched to fight a second invasion and lost the Battle of Hastings to William the Conqueror. So Harald Hardrada was responsible for England being conquered by the Normans! Sort of.