Adam of Bremen names "Suein Otto, filius magni Haroldi regis Danorum" when recording that he deposed and expelled his father. Converted to Christianity with his father, he was baptised "OTTO SVEND" in honour of Emperor Otto. Snorre records that "Svein, King Harald's son" rebelled after his father refused to share the kingdom with him, but was forced to flee, although his father had been mortally wounded and Svend was chosen as king after he died.
He rebelled against his father, who had refused to divide the kingdom with him, and forced him to flee to Jomsburg (now Wolin), succeeding in [early 987] as SVEND I "Tveskæg/Forkbeard" King of Denmark. King Svend immediately set about restoring the heathen religion in Denmark. Adam of Bremen records that King Svend persecuted Christians in Denmark. Sigvaldi Jarl at Jomsborg in Vindland captured King Svend and took him to Jomsborg, threatening to deliver him into the hands of the Wends unless he made peace. Adam of Bremen records that "rex Sueonum Hericus" invaded Denmark and expelled King Svend, who was eventually received by "rex Scotthorum" with whom he stayed seven years until the death of King Erik, after which he returned to Denmark after 14 years exile, although his period of absence appears exaggerated. King Svend first attacked England in 994, in the company of Olav Tryggveson (who succeeded in  as Olav I King of Norway). The Anglo-Saxon Chronicle records that "came Anlaf and Swein to London with ninety-four ships and kept up an unceasing attack on the city" 8 Sep 994. The Chronicle of the Princes of Wales records that "the isle of Man was devastated by Swain son of Harold" in 994. At that time, Stenton suggests that there was some support to declare Svend as king of England from those who despaired of the government of King Æthelred "the Unready". Adam of Bremen records that Olof King of Sweden invaded Denmark and expelled King Svend, but allowed him to return to his kingdom because "matrem suam habuerit". Olav I King of Norway attempted to invade Denmark but was defeated by King Svend in a naval battle "inter Sconiam et Seland", during which King Olav was drowned, after which Svend imposed himself as king of Norway.
Danish raids on England intensified after King Æthelred ordered the massacre of Danes in England 13 Nov 1002, which included the death of King Svend's sister Gunhild sister of King Svend. A full-scale Danish invasion came in 1013 and by the end of the year Svend had become SVEIN de facto King of England. King Æthelred fled to Normandy after Christmas 1013, but after Svend's death in Feb 1014 he was invited back, but on condition he improve his rule. Adam of Bremen records that King Svend held England for only a short time before he died. Guillaume of Jumièges records that “Danamarchæ...regique Sveno” invaded England but died “apud Lundoniam”, was buried “apud Danamarcham”, and succeeded by “filius eius...Chunutus” who launched a new invasion with “Lacman equidem Suauorum et Olavum Noricorum”.
The name and identity of King Svend's first wife is uncertain, although she seems to have been known as GUNHILD in Denmark. According to Snorre, Gunhild died before King Svend's second marriage but no precise date is given. Two possibilities emerge from the primary sources:
(1) --- of Poland, daughter of MIESZKO I Prince of Poland & his second wife Dobrava [Dobroslawa] of Bohemia. The sole authority for this appears to be Thietmar who refers to the mother of "filiis Suenni" as "Miseconis filia ducis, soror Bolizlavi successori eius", commenting that "long exiled by her husband…this woman suffered no small amount of controversy" although this comment is in no way explained.
(2) --- of the Wends, daughter of BURISLAV King of the Wends & his wife ---. Snorre names "Geira, Gunhild and Astrid" as the three daughters of "in Vindland…a king called Burizleif". In a later passage, he records Gunhild's marriage to Svend arranged as part of the peace deal brokered with the Wends by Sigvaldi (Jarl at Jomsborg) who was already married to another daughter of King Burizleif.
The matter is further complicated by the secondary sources. Brenner names King Svend's first wife "Gunhild of Poland", stating that she was the widow of Erik "Segersäll/the Victorious" King of Sweden, which appears to confuse her with King Svend's second wife Sigrid.... According to Europäische Stammtafeln, King Svend had a single wife "Gunhild of Poland", widow of King Erik of Sweden. In another table, Europäische Stammtafeln further confuses the issue by naming the second wife of King Erik of Sweden and King Svend as "Šwiętosława [Gunhild] of Poland" (although the basis for proposing this Polish name is not known) and the first wife of King Erik as Sigrid Storrada.
The only safe conclusions are: firstly, that King Svend did marry before marrying Sigrid, as a first marriage at the age of about 35 after the death of Sigrid's first husband seems improbable; and secondly, that his first wife was probably of Slavic origin from the southern Baltic coastal area, confusion in the primary sources between Polish and Wendish in this context being understandable. According to Ronay, King Svend divorced his first wife who returned to Poland taking her son Knud with her, but he cites no primary source to support this statement. 1