Sweyn Forkbeard (Sweyn I)(959-1014 AD) was king of Denmark, part of Norway, and briefly king of England. Initially king of Denmark and parts of Norway, Swyen engaged in repeated raids of the English coast in response to St. Brice's Day Massacre, ordered by Ethelred the Unready, in which many Danes including Swenn's sister were killed. Sweyn later invaded England, but only ruled England for a few weeks before his death. Ethelred resumed the throne.
Reference: “Anglo-Saxon Chronicle” trans. Anne Savage