Roberta Frank's 1984 paper is pretty scathing.
She quotes a 1974 talk by Afred Smythe which really laid the modern verison of the case out:
Examples of this practice many have included: King AElla of Northumbria, Halfdan son of King Haraldr Harfagri of Norway, King Edmund (a victim, like AElla, of the Great Danish Viking Ivarr), King Maelguala of Munster, and just possibly Archbishop AElfheah. ......... It happend in Scandinavia, in Ireland and in England. I am presuming that Francia was not exempt.
She then lays out the main claimed victims:
Ella and Halfdan - but points out that "the accounts of Saxo and the sagas are contradcited - sometimes flagrantly - by contemporary sources"
Lyngvi and the giant Brusi - no Brusi bonus for guessing these are figures of legend.
The actual procedure itself also becomes increasingly elaborate over time:
Saxo and
Ragnars saga report it is the scratching of the picture of an eagle on Ella's back.
Orkneyinga saga describes it as the tearing out of the ribs and lungs as an offering to Odin.
Pastr af ragnar sonum says the eagle is carved on the back and the ribs torn form the spine and then their lungs are pulled out to from the "wings"
This seemingly gained even more elements in the 19th century and draw everything together and the 1970s say the addiiton of Kings Edmund and Maelguala to the list of victims and the addition of extra elements like piericng with arrows and javelins, lying them face down on a stone and then beheading at the end to form a full an lengthy ritual.
Although the Sagas aren't considered to be historical reports people have believed that the tales of Ivarr performing the blood eagle were true as the Sagas quote an earlier source - half a stanza/12 words.
She then rattles on discussing how skaldicists are now aware that the authors of the sagas may have misinterpretted earlier accounts and been misinterprtted themselves in turn.
The stanzas themselves are not easy to unravel - a word for word translation gives:
And Ella's back,
at had the one who dwelt,
Ivarr, with eagle,
York, cut.
This leads to a little discussion of translation which results in things like:
And Ivarr, who dwelt at York, cut an eagle on Ella's back
or:
And Ivarr, who dwelt at York, had an eagle cut on Ella's back
As these translations were Englishified they became:
And Iwar that ruled at York cut an Eagle on the back of AElla
and
And Ivar, who dwelt at York, carved the eagle on AElla's back
although other possible interpretations are:
And Ivarr, who dwelt at York, had Ella's back cut by an eagle
Ella fell in battle against the Vikings at York and the line may purely refer to the fact that the Vikings had reduced his body to carrion (like Anthony and Cleopatra IV, vii 12).
There is also a tradition of this kind of imagery in skaldic poems of the 9th to 11th century e.g. "Torf-Einarr muses over whose 'lot it will be to stand under the eagle's claw'" and raven's claws are also thrown into the mix as well as eagles.
She then discusses the use of "the past participle with instrumental dative" which I'll skip
but she gives numerous examples of this.
The paper them examines Smythe's claims that King Edmund was also blood eagled - it certainly reads like someone making the text fit their theory.
Overall it seems pretty conclusive (if pretty dense at some points) although I'm sure debate goes back and forth as Vikings are redeemed and then damned but the grounds for the whole thing seem shaky indeed.
Also from an UL perspective it is interesting to see how the accounts become increasingly elaborate over time.
