[Disclaimer: It’s worth noting that, as the Wiki article puts it, “there is a continuing debate about whether the ritual (outlined below) was a literary invention, a mistranslation of the original texts or an actual historical practice.” There are a few sources that mention specific instances where it was used, but there isn’t quite enough evidence to be conclusive. So it may very well have been used in ritual sacrifice to Odin, for example, on a few occasions, but take it with a grain of salt!]
Like the rest of the world at that time, the Vikings were very cruel. One of the ways they liked to kill their enemies was by a hideous method called “the blood eagle.” The victim’s back was split open and his blood-covered lungs were lifted out so that they flapped around like a pair of wings as he struggled to take his least breaths.
Source:
Stephens, John Richard. “Eyewitness Reports.” Weird History 101: Tales of Intrigue, Mayhem, and Outrageous Behavior. New York: Barnes & Noble, 2006. 28. Print.
Further Reading:
It is a brutal yet effective tactic of psychological warfare. I wouldn't fuck with the guys who do that to their enemies. Didn't their enemies usually just pay them a yearly amount of goods to avoid being pillaged?