Kentaro Miura has a very well defined vision on how the Astral World functions and how it interacts with the physical world. Because it is so intricate I suspect he is projecting many of his views on how the world works into this.
First of all, there appear to be two major realms: astral and physical.
Within physical realm our bodies exist. Here we interact with matter and matter interacts with us.
The astral realm is the realm of the consciousness and (un-)consciousness. It is the kingdom of dreams, ideas, creatures of magic, spirits, angels/demons and eventually, Gods.
Sometimes, the astral and physical world overlap, producing paranormal phenomena, such as occurrences of spirits or creatures of magic and legends. This third, transitional area between those two is called the interstice. [211-212] According to Schierke, inside the interstice it is easier for the mind to affect matter. Its nature seems to be dynamic: the interstice may exist at any given time and location if certain requirements are met e.g. the presence of a branded person.
 There seem to be natural balancing mechanisms that are performed by mistletoe and Spirit Trees. These restrict the influence the realms have on the other. Neo-Griffith has destroyed many Spirit Trees in order to disrupt that balance and merge the realms, among them Flora’s Tree Mansion.
Properties of the Astral Realm
The astral realm seems to have different properties.
One property is atmosphere:
- Qliphot is an astral territory of evil. We also know what kind of evil it is, and it’s that of sexual perversion: this is the Troll’s domain after all, which creatures that enter the physical world to rape and pillage.  We can assume it is being ruled by Slan, given Skull Knight Calls her “Whore Princess of the Uterine Sea”.  Slan also appears during the orgy in the Heretic’s cave.
- Spirit Trees could be astral territories of good, because they are responsible for the natural balance between astral and physical realm. The big Cherry Tree on Elfhelm may be such a tree.
-  Schierke comments about astral regions with a different atmosphere, i.e. there are astral regions that go beyond just good and evil.
Another feature is the depth:
- Apostles, the God Hand and “transcended” Ganishka are contained in different depths of the astral realm.
- There appear to be different layers and areas situated in different depths of the astral realm. This goes with some limitations: You can only interact with realms that are close to your own layer: before his birth as Neo-Griffith, Femto could only interact with the physical world indirectly through dreams of the humans or apostles, e.g.  when he appeared before the King of Midland and  when he dueled Zodd in a “daydream” to gain his fighting force (which was done in a similar fashion to how he obtained Guts back in the Golden Age, by the way)
- With all considered in mind, a Behelit therefore enables for interaction between the physical world and deeper layers of the astral world, e.g. The Abyss.
Astral Realm and Consciousness
Because the mind is connected to the astral realm, that means every creature in the physical realm must have both an astral and physical body.  This is also how Schierke is able to enter the mind of Berserker Guts via a luminous body. The Kushan modified creatures and infused them with a different spirit, which implies animals possess an astral body as well.
As a consequence, the fact an astral body exists for a creature or human being implies the existence of a soul or mind as an entity. The evil spirits that follow Guts at night are said to be body-less souls roaming the land and  Neo-Griffith is also capable of summoning or retrieving souls of fallen soldiers. That implies the soul or consciousness of a dying one must have separated naturally from its original body. That also means there are “evil” and “natural” wraiths, because these wraiths are depicted as white lights, rather than the evil black ones we see haunting Guts at night.
It is also possible for the mind (soul?) to leave the physical body to enter another physical body without dying.  Alternatively, one can venture into the astral realm to summon powerful spirits by making a contract with them, lending their power as a result. This is very similar to modern day occultism or necromancy.
Rules of Possession
How possession works is in coherence with the concept of the astral and physical realms. When one is possessing another creature, the possessor takes control over the physical body of its host. It is like the astral entity, the “mind” is being replaced with another, an “external” one. What also happens is that the host’s appearance is changed during this process.
There are quite a few instances in Berserk where this happens:
- Each time the Berserker Armor possesses Guts without interference by Schierke. Guts’ appearance changes completely to that of a canine creature.
-  When Schierke allows Berserker Guts to keep his sanity, it is her possessing him instead, allowing him to maintain a degree of rational thought. On astral level, she is taking the place of the armor instead, while the Berserker Armor retains some control over his physical body, as it is still fully covering up his limbs. Guts’ appearance is also slightly different, with his face below the Berserker Armor helmet showing.
- When Femto descends from the astral plane and enters his reincarnation vessel, the Egg Apostle, he also possesses the human baby inside of it during the Birth Ceremony. Femto changes the appearance of his host’s body (the Moonlight Boy’s) to Griffith. This perhaps seems like a crazy theory at first glance, but it has a solid basis. You can read more about this in Demon Infant, Moonlight Boy and Griffith.
- The Kushan turned tigers, crocodiles, whales and other animals into living weapons of war. Because of their different appearance and the fact they are infused with a different spirit, it also appears some kind of possession. These familiars return to their original form when slain.
- Fascinating: this also happens to Apostles. Wyald’s form returned to that of an old man once his apostle body was sucked into the Abyss. Something similar happens to the Snail apostle. This could imply that Apostles are also possessed, however, their possession by evil powers is voluntary (they had to make a sacrifice to obtain them). It also looks like their souls vanish into the Abyss after their death, because of their pact with evil powers.
It looks like possession is not limited to evil powers either: we have Wyald who can change into his apostle form at will, we have Rosine, who is capable of controlling how human she looks to look less frightening towards Jill. But then we also have “good” characters such as Isma the mermaid and Dannan, who are both able to transform to their true form, presumably using their real name Ivelera mentions during the Sea God Fight.
It seems like the concepts of possession by an astral power (whether it is evil or not) and transformation into a stronger form are tightly interwoven.
It is most fascinating how detailed and coherent Miura’s writing in this regard is!
Thank you for reading. Let me know what you think about this topic in the comments section below!
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