What is the Demon Infant?
In order to understand the relationship between the Moonlight Boy and Griffith, we must take a look the origins of the demon infant and how he managed to fuse with Griffith.
Originally, the demon infant was Guts and Casca’s son conceived out of love, but its fetus was corrupted when Femto raped Casca during the Eclipse.
The demon infant took care of Guts during Black Swordsman and Lost Children Arcs (at least… I think it attempted to communicate how far off Guts has driven from being his real self).
Later, we know the demon infant was following Casca after she ran off to Albion. Since the infant was protecting her all the way, towards the beginning of the ceremony, all of its energy was spent. Dying, it was then picked up by the Egg of the New World.
There it appears like the Egg of the New World was the “womb”, and the demon child a “carrier” aka the actual body.
I also want to point out how the imagery here from volume 20 is very similar to the Corridor of Dreams.
Now from the Eclipse we know that you need some kind of organic matter (from those you sacrificed) in order to become apostle, which will fuse into the person sacrificing, forming a new body. Griffith describes this during the eclipse as: “All their deaths… are piercing through me”.
Would it be possible for a God to be reborn out of nothing but just a womb? I mean even Femto required Griffith’s real body to be formed to what he is. So the answer to that is a potential no. Judging by the visuals alone, it looks like the Moonlight Boy is the actual body of Griffith, and Griffith the spirit that inhabits said body. Is that what Griffith meant when he contemplated how the infant merged into his vessel…!? We can already see that how exactly the demon infant relates to Griffith is still up to interpretation.
So it seems like the demon infant’s body in part was used for this process. This begs the question whether they are actually fused together or if it is Femto possessing the child. This might make a huge difference towards the finale, because possession might mean they will be able to release their kid. For a fusion it might turn out super tricky.
When the Egg of the New World picked him up, the kid defied death (just like his father) and because of this is possibly more an astral being existing on the interstice, a being that (probably) has more resemblance to Skull Knight. We also know the demon infant can be quite powerful, especially when protecting his parents.
We know the demon infant has some influence over Griffith’s actions, given that he saved Casca from falling boulders later on the Hill of Swords. The demon infant’s presence inside of Griffith is presumably the reason why Casca is drawn to him – in the same way she is drawn to the Moonlight Boy.