Sweyn II Estridsson Ulfsson was the King of Denmark from 1047 to 1074. He was the son of Ulf Thorgilsson and Estrid Margarete Svendsdatter, daughter of Sweyn I of Denmark and sister of Canute the Great. Because of his relationship to Canute the Great he was a pretender to the throne from his early years. When king Harthacanute died in 1042, he claimed the Danish throne, but lost to Norway's King Magnus, who made Sveyn a jarl (earl or viceroy) instead.
He soon rebelled against Magnus and had the Danish nobles crown him king, but was defeated by Magnus and fled to Sweden. The war between Magnus and Sweyn lasted until 1045, when Harald Hardråde returned to Norway from exile. Harald and Sweyn joined forces and forced Magnus to share the throne with Harald. In 1047 Magnus died, having stated on his deathbed that his kingdom would be divided up: Harald would get the throne of Norway, while Svend would be king of Denmark. Harald, unwilling to relinquish Denmark, now attacked Sweyn and fought a long war which dragged on until 1064, when Harald relinquished his claims to Denmark. After Harald was defeated and killed at the Battle of Stamford Bridge and William the Conqueror in turn defeated Harold Godwinson, Sweyn turned his attention to England, once ruled by his uncle Canute the Great. He joined forces with Edgar Atheling, the last remaining heir of the Anglo-Saxon royal house, and sent a force to attack King William in 1069. However, after capturing York, Sweyn, accepted a payment from William to desert Edgar, who then returned into exile in Scotland. Sweyn's first marriage was to a girl to whom he was distantly related, and the pope ordered that he dissolve the union, which he did, only to take one mistress after another during the rest of his life. Sweyn fathered at least 19 children, probably more, and while none of them were born in wedlock, and none of their mothers are known, five of his numerous sons became kings after his death in 1074:
- Harald III (Harald Hén) d. 1080
- Canute IV the Saint (Knud den Hellige) d. 1086
- Oluf I (Oluf Hunger) d. 1095
- Eric I (Erik Ejegod) d. 1103
- Niels d. 1134