Sweyn Forkbeard
In 1013 Sweyn Forkbeard, already the king of Denmark and of Norway, overthrew King Æthelred the Unready of the House of Wessex. Sweyn had first invaded England in 1003 to avenge the death of his sister Gunhilde and many other Danes in the St. Brice's Day massacre, which had been ordered by Æthelred in 1002.