Sweyn I Forkbeard was king of Denmark and Viken between 986 and 1014. He briefly ruled England in winter 1013-14 before his death. Much of the sources for his life are contradictory.
Ruthless, he rebelled and probably murdered his own father for the Danish throne. Although his father had Christianised Denmark Sweyn was damningly criticised by chroniclers at Bremen-Hamburg for being a pagan, though actually he built a wide number of churches throughout his lands whilst courting English priests. He, like many of his successors, understood that continued dominace of the Danish and Scandinavian church by Bremen would lead to increasing dominance by the Holy Roman Empire. This was a trend to be resisted as much as possible.
His reign in Viken did not go unchallenged and between 995 and 1000 it was effectively ruled by Olaf II Tryggvasson who had repeatedly fought Harald I for the throne. Olaf married Sweyn's sister Tyra suggesting Sweyn may have come to terms with him. However Olaf would die at the Battle of Svolder in defiance of Sweyn. Sweyn quickly mopped up the remaining Vikene lords and reclaimed Viken for himself and his heirs.
Sweyn invaded England in 1004 in retaliation for the St. Brice's Day massacre which may or may not have involved the death of his sister. From the initial raids to full blown conquest in 1013 Sweyn consistently beat the English forces of Aethelred II and was repeatedly bought off by vast amounts of Danegeld. For most of the invasions Sweyn delegated to other warlords such as Thorkell the Tall, but in 1013 sought all out conquest himself. He was crowned King of England by the English nobles on Christmas Day 1013 but would die only a few weeks later.
On his death his realms were divided and England recalled Aethelred from Normandy. Harald II took Denmark while Cnut would be forced to raise an army to continue the war against England. Cnut would in time rule a huge sea empire, however it would actually fall to Sweyn's daughter Estrid to found the dynasty which would turn Denmark, and Anglia, into a true power.