King of Denmark, possibly husband to the widow of King Erik of Sweden (and thereby claimant to its throne), and also claimant to rule over Norway, Svein (known as "Forkbeard", referring to a pitchfork-shaped beard he wore) came first to England's notice as a raider, or king of raiders, being blamed for the series of Viking attacks on England which followed the decision of Aethelred Unraed to have all Danes in England massacred in 1002CE.
In 1013 Svein himself led the fleet and mounted a full-scale invasion. He began at Sandwich in Kent, swept up the east coast to Northumbria to obtain the sumission of the earl, Uhtred, also obtaining the sumission of other east-coast shires and hostages from each. He then switched to land, requiring the English to horse his army, attacked and obliged Oxford and Winchester to submit, and rode on London.
The Londoners tore down the bridges and resisted, and Svein was obliged to fall back. Aethelred had, however, fled, to Normandy, and, alone and isolated, London submitted, Svein entered, and was crowned king on Christmas Day. He then moved to Gainsborough and was in the process of organising his fourth kingdom when, in February 1014, he died.
Svein had a son to take over Denmark for him, and the Danish army declared his younger son, Canute, king of England. However, he and they returned to Denmark, and Aethelred returned and was restored in England.