I’ve written about the Beast of Darkness before, but I think I can create a much more accurate picture of what it really is as my understanding of it grew.
The following assertions are based on Schierke’s statements pieced together to make sense. Relevant chapters are 201, 215 and 251.
First, in chapter 215 Schierke states creatures of nightmare and fantasy are a product of human consciousness. Where they manifest, whether it is the World of Idea or the World of Thought—sections of the Astral Realm that are both brought up in chapter 201—is not entirely clear. What is clear that the Beast of Darkness belongs to this category, since it occurs in Guts’ nightmares first before affecting his actions (this happens in chapters 187 and 188).
Now, there is another piece of information that is relevant: in chapter 251, Schierke describes the means magic can be practiced with, namely to tangibly imagine something in one’s mind. She says that the Astral World composes of wills and powers. In order to make use of them effectively, it is important to retain someone’s ethereal body and it should be a reflection of their physical body. It’s interesting both the roaming souls of the dead (who died with malice) and the Beast of Darkness fit the description of a “will or power”. They can both possess someone, too, should the person lose their sense of ego. It almost happened to Schierke who got caught up in the Lady of the Depth’s power at Enoch Village. It also happens to Guts every time he is confronted with entities that attempt to possess him, be it Beast of Darkness or otherwise. I suppose that also makes Guts an “involuntary magic user”.
Now to sum it up, if the astral world composes of wills and powers on one hand, and creatures of nightmare and fantasy originating from human consciousness on the other, then what does that make the Beast of Darkness?
Is the the Beast of Darkness Guts’ will that started to exist as body of though inside of the Astral World?
As such, it closely belongs to Guts. It formed inside of the astral world starting from the moment two branded people, Guts and Casca, traveled together. Two branded people essentially create an even larger distortion that was so strong they were potentially one of the many catalysts for the Birth Ceremony. The interstice that forms around them should be even more severe than usual, too. In this situation, the “mind affects matter” rule comes into play, with Guts forming his Beast of Darkness inside of the Astral World.