I recall reading, perhaps in one of the Hannibal Lector books, mention of the Viking blood angel.
The slain enemy was cut under the scapula and the lungs retracted and spread out like bloody wings.
Probably just the author’s fictional device...probably.
Blood Angel may have been the author’s fictional name for something more real, the blood eagle.
From Wikipedia: The blood eagle is a ritualized method of execution, detailed in late skaldic poetry. There is an ongoing debate about whether the ritual was a fictional literary device, a mistranslation of the original texts or an actual historical practice.