I believe this is a reference to the Blood Eagle practice. There is in fact a huge ongoing controversy over whether it was real.
On the pro side, it is attested in three old sagas: The Orkneyinga, Heimskringla, and Norna-Gests and there's probably a reference to it in The Tale of Ragnar's Sons.
On the minus side, both of those first two sagas refer to the same incident, so it could well have been a one-off that was thought up on the spot. The third saga was not written down until the Medieval period was in full swing. The "blood eagle" isn't attested in any of our oldest and best sources (eg: The Eddas). All of the references we do have came well after Christianization, and Christians had a vested interest in portraying pagans as barbaric. Particularly literate Christians, who tended to be associated with the Catholic Church.
