The evidence for the reality of this form of execution is actually extremely scanty, as I discussed in this earlier thread, referencing an earlier essay on the subject that I wrote a few years back for the Smithsonian.

That for the carving of the blood eagle on King Ælla boils down to how you interpret a couple of lines of rather gnomic skaldic verse dating to the reign of Cnut, about 150 years after the fact.

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