In addition to the "Anglo-Saxon Chronicle", Ælla is also mentioned in Scandinavian sources, such as the Norse sagas. According to the latter, Ælla captured the semi-legendary Swedish-Danish Viking leader Ragnar Lodbrok and put him to death in a pit of snakes. The historical invasion of Northumbria in 866 occurred in retaliation for Ragnar's execution, according to "Ragnarssona þáttr" ("The Tale of Ragnar's Sons"). While Norse sources claim that Ragnar's sons tortured Ælla to death by the method of the blood eagle, Anglo-Saxon accounts maintain that he died in battle at York on 21 March 867. Concerning the Norse claim, Roberta Frank reviewed the historical evidence for the ritual in her "Viking Atrocity and Skaldic Verse: The Rite of the Blood-Eagle", where she writes: "By the beginning of the nineteenth century, the various saga motifs—eagle sketch, rib division, lung surgery, and 'saline stimulant'—were combined in inventive sequences designed for maximum horror." She concludes that the authors of the sagas misunderstood alliterative kennings that alluded to leaving one's foes face down on the battlefield, their backs torn as carrion by scavenging birds. If this is to be believed, then it is easy to surmise that the mention of his death via the blood eagle is in fact a description of his death on the battlefield, which would make both accounts of his death consistent.