[quote=@Guess Who]
I've been hoping since the beginning that, whoever he is, he's some character that we've seen before in the comics. Considering that literally every other character in the cast was made specifically for the show I know that's unlikely now, but I still want to hope it's the Shadow King.
[/quote]

Well each character is based on elements of comic book characters, gives them the advantage of altering the source material to fit the show without enraging fans of traditional comic storylines. With that in mind the Devil with the Yellow Eyes is probably based on the concept of the Shadow King without using the entity itself.