Though the anecdote about the werewolf-prince is but a small part of the epic, Nabokov was surely aware of the greater corpus of Vseslav tales. In a 1950 letter from Nabokov to Szeftel, Nabokov wrote that he was “looking forward to Vseslav,” by which he meant that he was looking forward to reading Szeftel and Jakobson's article, which had recently appeared in Slavic Studies (Diment 108). In the article, the authors collate and analyze all of the known narratives concerning the magical prince, noting that the name Vseslav “is rare in Russian tradition” (303). Jakobson and Szeftel show that “the whole context of the Vseslav epos plainly indicates that it is a werewolf story” (356). Surely Nabokov had this very Vseslav in mind when he gave Charles the Beloved that “rare” name.
Here is a link to the article, though some of it is unavailable:
The mention of Vseslav in the Song of Igor's Campaign is very
brief, it is even not clear whether the poet speaks of a werewolf
or it is just a poetic metaphor. Does it exist any special
epos about Vseslav (sorry, I didn't read Jakobson and Szeftel)?
Or it is all about this short mention in the Song of Igor's...?