There are a couple of theories floating around trying to reconstruct the events surrounding King Gaiseric/Skull Knight and his ancient rivalry with Void. Here is how I think it may have happened.
Claim #1: Skull Knight sacrificed himself in the Berserker Armor to protect his kingdom from the God Hand.
If you look at Skull Knight ‘s sword and shield they all have roses and thorns on them and the rose is a symbol of self sacrifice in heraldry (Protip: ctrl + f and look for “rose”).
Skull Knight’s sword has thorns as hilt and a rose-like shape worked into the grip where it connects to the hilt. His shield also possesses an engraving of a rose surrounded by thorns.
Miura did his research: the symbols he uses in his flag designs are very fitting for the group or faction they represent. There is all the reason to think the symbolism applies to this specific character as well. (An article about more examples of heraldry in Berserk is yet to be written…).
Skull Knight’s current form:
Puck notes before the Tower of Conviction that Skull Knight gives him elf-vibes. This implies that while he is some kind of powerful astral creature, there is an important distinction to be made compared to Apostles. Given how Guts currently resists the Berserker Armor and succeeds maintaining part of his consciousness while going berserk, it is possible something similar happened to King Gaiseric. Flora in volume 24 comments that she wants to believe that Skull Knight still has his human heart:
A possible reconstruction of events:
After Void sacrificed, an event similar to the Eclipse must have happened. Perhaps even King Gaiseric was offered as sacrifice and thus branded. Perhaps he attempted to resist the God Hand and kill as many apostles as possible and perhaps to also rescue those who were being sacrificed somehow. As this happened, he must have used the Berserker Armor along the way.
As Gaiseric approached the limit of his sanity, he was still resisting the Berserker Armor as it attempted to completely take over him, transforming him not into an Apostle, but rather into a elf-like astral being. It appears for what reason you sacrifice and whether you sacrifice yourself or not determines the outcome: Maybe Skull Knight sacrificed himself for someone else’s sake (his kingdom and his people), realizing the mistakes he has done while he was leading it as King. He became the astral being he is, rather than sacrifice someone for his own selfish goals and become either an Apostle or a God hand member as a result.
Claim #2: Void was the Sage Mozgus describes during the Rebirth scene and it was also him who did the sacrifice.
Void was tortured like Griffith was because he criticized King Gaiseric for letting his kingdom sink into corruption and poverty (more of this in this article). This implies Void cared for that kingdom enough to make it suitable as sacrifice. By the sound of it, Void might have been a royal advisor trying to prevent the kingdom going towards its certain doom and was incarcerated. He perhaps spoke up against certain policies or even was involved in an intrigue to overthrow the King.
Interestingly enough, if we attempt to detect parallels in the current line of events, the one advisor who is suspicious of his (future) King Griffith and known to craft intrigues is Minister Foss.
Claim #3: Void being the first God Hand member makes sense if we do the math and there is one unknown entity responsible for picking the first member.
If we do the math and assume every 216 years a God Hand members ascends to their ranks, Void must have been the very first one given that 5 God Hand members X 216 years is 1080 years, which is around the time Gaiseric’s empire existed. If Void was the first God Hand member, who was Void sacrificing to?
Correction: The above calculation is not correct. If we go back 216 years for every God Hand member Void’s date of birth was actually 864 years ago, not 1000. Still, the question who Void sacrificed to still stands. Was there some kind of “prototype” God Hand member? And what if Void wasn’t the first, but the second…? Was Skull Knight supposed to be the first God Hand member, but it didn’t work out for some reason? Something seems definitely off here and we might be in for a surprise (e.g. an additional character who is yet to be revealed).
Void was tortured just like Griffith was and according to Mozgus, called one angel for help, presumably in a moment of despair. Was it an actual angel (as in, a being similar to the Lords of the Four Directions), an actual demon, or perhaps even a manifestation of the Idea of Evil? I believe Miura wanted to keep his options open here, which is why the chapter of Griffith talking to the Idea of Evil was removed (with this it is technically non-canon).
Because the information about this topic is really sparse, we cannot derive much from it. I really hope we get a bit of additional information on this topic, sparse enough to keep the mystery of the characters involved intact – but enough to make a more solid hypothesis.