I do have several theories about the Skull Knight. I think he was that supreme king but I don't think he was 'human' I think he might of once came from the place as Puck. Because Puck said he gives off the same sensation from his homeland. And I also think he and Void were once best friends like Guts and Griffith. It mentioned that 'a man in a tower' prayed to four or five angels that stopped the king from becoming to powerful, but I think the king most likely killed 'that' god hand and Void took the mantle of being the first 'angel' of his god hand. Because Skull knight said that he will be the one that will kill Void.