This may have already come up...but what if Skull Knight is an apostle who was reborn into the flesh, took his own kingdom, and then got shafted by the remaining apostles after he had it? Perhaps the path Griffith is on currently. I do seem to remember something about an apostle being reborn into flesh every 1000 years, though I can't remember the source.
Or...I may be completely off. Your call.
Or...I may be completely off. Your call.