Berserk is a story about a 1080 year long cycle in human history where god-like creatures are made and destroyed, with the ultimate purpose of pushing the entire planet along the path of maximum suffering. During this millenial cycle, five Eclipses occur, evenly spaced apart by 216 years apiece. During an Eclipse, a King Behelit may be used to sacrifice one's humanity in exchange for "limitless" power and a position among the God Hand, a cadre of up to five elite pandimensional Eldritch abominations that answer directly to the only known true god of the Berserk world.
Currently, there are five members in the God Hand, but there were only four before Griffiths made his sacrifice during the most recent Eclipse. This gives the impression that somewhere around 1080 years ago there should have been zero members in the God Hand (assuming no one ever turns down the membership offer).
But, we've also been told that there was a kingdom ruled by Gaiseric that was destroyed 1080 years ago by five angels. So, who the hell were they? Excluding Griffiths, even if the current four other members of the God Hand were the ones who destroyed Gaiseric's kingdom, then who was the fifth one?
It's a common theory that Gaiseric and the Skull Knight are the same person. Keep in mind what Purple Hermit's said, though: Legends can get changed or misconstrued over time. The current people of the Berserk world have a legend about a falcon of light that will save humanity, but there have also been whispers of a falcon of darkness. And we're already well aware that the five "angels" that ruined Gaiseric were actually demonic horrors.
What if, instead of Skull Knight, Void is the one who used to be Gaiseric?
Perhaps, 1080 years ago, the story was very similar, almost identical, to what we're currently seeing. At the time, there were only four members of the God Hand, but they weren't the same as the ones we currently know. Still-human Gaiseric, who dreams of building a shining kingdom, greater than any in the history of the world, acquires the "King" Behelit and eventually has a fateful meeting with and befriends a man (let's call him Crane) who shows him over time that such dreams are folly. As irregularities in the world begin to grow and the Final Eclipse (the fifth one, once every 1080 years) approaches, Gaiseric and Crane have a falling out. Gaiseric makes a vital mistake and his dreams are dashed upon the ground, ultimately giving him enough of a grudge to happily sacrifice Crane and his own humanity to become the newest God Hand member, Void. With the God Hand complete, Gaiseric/Void returns to the material world to erect his kingdom. This is the kingdom that we've heard of in the legends, which is said to be rivaled by modern day Falconia.
But, and here's the rub, Crane survived the attempted sacrifice (probably saved by the previous cycle's protagonist). Donning the Berserker armor, he slowly builds the strength of his own grudge and sacrifices his own mortality in order to eventually become strong enough to stop his misguided friend, Gaiseric. The Berserker armor ultimately teaches Crane how to make his bloodlust manifest like an apostle, at which point he no longer needs the cursed metal suit. After removing the armor with the help of the Elf King, Crane returns to Gaiseric's kingdom for the final showdown. For poetic justice, Crane dons Gaiseric's old armor ("The old you would have never wanted to become this!") which fuses to his body and turns him into the Skull Knight. The Skull Knight and Void do battle, the other God Hand members are called for support. Chaos ensues, and all of the previous God Hand members are killed in the process. The entire kingdom is flattened in the wake of their battle, but Void and Skull Knight survive the encounter, each one not having quite enough power to end the other.
Well, I don't know. It could be a neat idea…? I like the wolf imagery associated with Guts, so I don't know how I'd feel about him becoming the dark falcon that takes down the falcon of light, but I don't think it's completely implausible.
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And if Void really is supposed to be the sole survivor of the previous God Hand, then it would only take 1080-216 years to make the new set complete, so this can't be completely right. Maybe people do turn down the option to become a God Hand member more often than I think. Or maybe the God Hand members aren't as immortal/invincible as they claim to be.