The blood eagle is a ritualized method of execution, detailed in late skaldic poetry. There is a continuing debate about whether the ritual was a literary invention, a mistranslation of the original texts or an actual historical practice.
There are only two incidents and one oblique reference in Norse literature which mention the ritual. The primary versions both have some commonalities: the victims are both noblemen (Halfdan Haaleg or "Long-leg" was a prince and Ælla of Northumbria a king) and both of the executions were in retaliation for the murder of a father.
Einarr & Halfdan
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Einarr made them carve an eagle on his back with a sword, and cut the ribs all from the backbone, and draw the lungs there out, and gave him to Odin for the victory he had won.
Afterwards, Earl Einarr went up to Halfdan and cut the "blood eagle" on his back, in this fashion that he thrust his sword into his chest by the backbone and severed all the ribs down to the loins, and then pulled out the lungs; and that was Halfdan's death.
Ivar & King Ælla
In Þáttr af Ragnars sonum, the "Tale of Ragnar's sons", Ivar the Boneless has captured king Ælla of Northumbria, who had killed Ivar's father Ragnar Loðbrók. The killing of Ælla after a battle for control of York, is described thus:
They caused the bloody eagle to be carved on the back of Ælla, and they cut away all of the ribs from the spine, and then they ripped out his lungs.
The blood eagle is referred to by the eleventh-century poet Sigvatr Þórðarson, who, some time between 1020 and 1038, wrote a skaldic verse named Knútsdrápa that recounts and establishes Ivar the Boneless as having killed Ella and subsequently cutting his back.
Sighvatr's skaldic verse in Old Norse:
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Ok Ellu bak,
And Ella’s back,
And Ívarr, the one
Skaldic poetry, a common medium of Norse poets, was intentionally meant to be cryptic and allusive, therefore the idiomatic nature of Sighvatr's skaldic verse, describing what has become known as the blood eagle, is a matter of historical contention. This is all the truer in this case, since, in Norse imagery, the eagle was strongly associated with blood and death.
Another oblique reference to the ritual appear in Norna-Gests þáttr. There are two stanzas of verse near the end of its section 6, "Sigurd Felled the Sons of Hunding", where a character describing previous events says:
Nú er blóðugr örn
There is debate about whether the blood eagle was historically practiced, or whether it was a literary device invented by the authors who transcribed the sagas. No contemporary accounts of the ritual exist, and the scant references in the sagas are several hundred years after the Christianization of Scandinavia. Roberta Frank writes in her article "Viking Atrocity and Skaldic Verse: The Rite of the Blood-Eagle": "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 which described carnivorous birds scavenging after battles, i.e. killing a foe and turning them into carrion on the battlefield.
David Horspool in his book "King Alfred: Burnt Cakes and Other Legends" (while not committing to the historical veracity of the ritual), compares the lurid details of the blood eagle to martyrdom tracts expressing the final tortures of worthy victims in terms reflective of the intended execution of Saint Sebastian (shot so full of arrows that their ribs and internal organs were exposed) which were combined and exaggerated into a grandiose torture and death ritual that never was. 
Ronald Hutton's The Pagan Religions of the Ancient British Isles: Their Nature and Legacy states (p. 282) that "the hitherto notorious rite of the 'Blood Eagle,' the killing of a defeated warrior by pulling up his ribs and lungs through his back, has been shown to be almost certainly a Christian myth resulting from the misunderstanding of some older verse."
If the procedure were performed, the condemned would die of suffocation very soon after the lungs were pulled out (since breathing occurs via the diaphragm and chest muscles) and would probably lose consciousness due to blood loss and shock before that.
- Frank, Roberta (1984). "Viking atrocity and Skaldic verse: The Rite of the Blood-Eagle" (PDF). English Historical Review (Oxford Journals) XCIX (CCCXCI): 332–343. doi:10.1093/ehr/XCIX.CCCXCI.332. Retrieved 30 March 2015.
- Tracy, Larissa (2012). Torture and Brutality in Medieval Literature: Negotiations of National Identity. DS Brewer. pp. 109–111. ISBN 9781843842880.
- Dash, Mike (18 March 2013). "The Vengeance of Ivarr the Boneless". http://www.smithsonianmag.com. Smithsonian. Retrieved 30 March 2015.
- Dasent, G.W. (1894). "Icelandic Sagas and Other Historical Documents Relating to the Settlements and Dsecents of the Northmen on the British Isles Vol III - The Orkneyinger's Saga". Rerum Britannicarum Medii Ævi Scriptores, Or, Chronicles and Memorials of Great Britain and Ireland During the Middle Ages (University of Minnesota: Great Britain. Public Record Office) 88 (3): xxvi, 8–9. Retrieved 30 March 2015.
- Hollander, Lee (1964). Heimskringla: History of the Kings of Norway (7th, 2009 ed.). Univ of Texas Press. p. 84. ISBN 9780292786967.
- Knútsrápra by Sigvatr Þórðarson, Skaldic Poetry of the Scandinavian Middle Ages
- Norna-Gests þáttr
- Hardman's translation of Norna-Gests þáttr
- Horspool, David (2006). King Alfred: Burnt Cakes and Other Legends. London: Profile Books. pp. 44–45. ISBN 067402320X.