The valravn does not appear in the Eddas, the sagas, or any medieval Norse text. It belongs to Danish folk song and to the folk-belief material that antiquarians and folklorists gathered in the eighteenth and nineteenth centuries, and that is a different kind of source with different problems. Ballads were sung, varied by singer and region, and written down late by collectors who chose what to record and sometimes tidied it.
What the ballads give is a raven made from a battlefield. A raven that feeds on the slain, in some versions on a king, acquires more than a bird should have, and the way back to human shape runs through blood: drinking a child's blood, or eating a heart, turns the creature into a knight. The horror is that the transformation works.
This entry is here partly as a control on the others. Norse mythology as it is popularly known is an assembly of medieval Icelandic manuscripts, later Danish and Swedish folk collection, nineteenth century national romanticism and twentieth century fantasy, and those layers are usually presented flat. The valravn is a clean case of the second layer, and its current visibility, on album covers and in tattoo flash and in Danish design, comes from the third and fourth. None of that makes it inauthentic. It makes it recent, and datable, which most of the material is not.