Danish sources
The Chronicon Roskildense (c. 1138) mentions Lodbrok (Lothpardus) as a father to the utterly cruel Norse King Ywar (rex crudelissimus Normannorum Ywar) and his brothers, Inguar (a double of Ywar), Ubbi, Byorn and Ulf, who rule the northern peoples. They call on the various Danish petty kings to help them ruin the realm of the Franks. Ywar successfully attacks the kingdoms of Britain, though not as an act of revenge as in the Icelandic sagas. The chronicle of Sven Aggesen (c. 1190) is the first Danish text that mentions the full name, Regnerus Lothbrogh. His son Sigurd invades Denmark and kills its king, whose daughter he marries as he takes over the throne. Their son in turn is Knut, ancestor of the later Danish kings.
Neither of these sources mentions Ragnar Lodbrok as a Danish ruler. The first to do so is Saxo Grammaticus in his work Gesta Danorum (c. 1200). This work mixes Norse legend with data about Danish history derived from the chronicle of Adam of Bremen (c. 1075). Here Ragnar's father Sigurd Ring is a Norwegian prince married to a Danish princess, and different from the victor of Brávellir (who had flourished about thirteen generations earlier). Sigurd Ring and his cousin and rival Ring (that is, Sigfred and Anulo of recorded history, d. 812) are both killed in battle, whereupon Ragnar is elevated to the Danish kingship (identified by Saxo with Ragnfred, d. 814). His first deed is the defeat of the Swedish king Frö, who has killed Ragnar's grandfather. Ragnar is assisted in this by a ferocious shield-maiden named Ladgerda (Lagertha), whom Ragnar forces to marry him after killing a bear and a great hound to win her hand. In this marriage, he sires the son Fridleif and two daughters. Ragnar later repudiates his marriage to Ladgerda and marries Thora Borgarhjort, a daughter of the Swedish king Herrauðr, after killing two venomous giant snakes that guard Thora's residence. His sons with Thora are Radbard, Dunvat, Sigurd Snake-in-the-Eye, Björn Ironside, Agnar and Ivar the Boneless. From a non-marital relationship with an unnamed woman (described only as the daughter of a man named Esbjørn), Ragnar fathered Ubbe. Another, final marriage to Svanlaug (possibly another name for Aslaug) produces another three sons: Ragnvald, Eric Weatherhat and Hvitserk.
The sons were installed as sub-kings in various conquered territories. Ragnar led a Viking expedition to England and killed its king, Hama, before killing the earls of Scotland and installing Sigurd Snake-in-the Eye and Radbard as governors. Norway was also subjugated, and Fridleif was made ruler there and in Orkney. Later on, Ragnar with three sons invaded Sweden where a new king called Sörle had appeared and withheld the heritage of Thora's sons. Sörle and his army were massacred, and Björn Ironside was installed on the throne. Sometime later Björn was put in charge of Norway, while Ragnar appointed another son, Eric Weatherhat, as ruler in Sweden; he was subsequently killed by a certain Eysteinn. One of the sons, Ubbe, revolted against his father at the instigation of his maternal grandfather Esbjørn, and could only be defeated and captured with utmost effort. Saxo moreover tells of repeated expeditions to the British Isles, one of which cost the lives of Dunvat and Radbard. Ælla, son of Hama, with the help of allies known collectively as the Galli – possibly a group of Norse-Gaels (who were known in Old Irish as Gall-Goídil), expelled Ragnar's sub-ruler Ivar the Boneless from England and remained a persistent enemy. Finally, the Scythians were forced to accept Hvitserk as their ruler. In the end, Hvitserk was treacherously captured by the Hellespontian prince Daxon and burnt alive with his own admission. Hearing this, Ragnar led an expedition to Kievan Rus' and captured Daxon who was spared and exiled.
Unlike the Icelandic sources, Saxo's account of Ragnar Lodbrok's reign is largely a catalog of successful Viking invasions over an enormous geographical area. Among the seaborne expeditions was one against the Bjarmians and Finns (Saami) in the Arctic north. The Bjarmian use of magic spells caused foul weather and the sudden death of many Danish invaders, and the Finnish archers on skis turned out to be a formidable foe. Eventually, these two tribes were put to flight, and the Bjarmian king was slain. The historical king Harald Klak is by Saxo (based on a passage in Adam's chronicle) made into another persistent enemy of Ragnar, who several times incited the Jutes and Scanians to rebel, but was regularly defeated. After the last victory over Harald, Ragnar learned that King Ælla had massacred Ragnar's men on Ireland. Incensed, he attacked the English king with his fleet but was captured and thrown to his death in the snake pit – the fate ascribed by tradition to the early Burgundian king Gunnar, as recounted in the Icelandic sagas. In spite of all his praise for Ragnar Lodbrok, Saxo also considers his fate as God's rightful vengeance for the contempt he had shown the Christian religion.