When the Band of the Hawks was going to save Griffith as they went down the tower, they saw dead bodies but with the brand in their foreheads. They also tell the story of Emperor Gaeseric and how he united the kingdom, but was punish by God. Also when the Skull knight saves Guts and Caska he goes straight for Void after breaking the eclipse. I think that Void and Skull Knight might have known each other before midland was united and something happen between the two of them. Also the Skull Knight has the eyes of an apostles, could he himself be an apostle?, or somehow he gained the power of an apostle ,by collecting behelits and wearing the berserker armor, without losing his free will.