Souls, Spirits, and Demons

In the world of AoT, the soul, spirits, and demons have much in common, moreso than anyone in-universe actually knows or understands.

The Soul
A soul has, for thousands upon thousands of years, been an integral part in folklore and faith throughout the entire world. The concept of a soul appears in every (known) culture and society, although the exact nature of it differs. In actuality, the soul does exist. Its true nature is not known fully, but it is assumed that much of what makes a person - their personality - is a product of the persons soul. What actually happens to a soul when a person dies is up to debate, and opinions differ widely across the world. It is mostly assumed that it passes on to some kind of afterlife: wether this afterlife is a paradise or a realm of torment was unknown until the end of the Age of Turmoil, where it was discovered that souls end up in the Arcane. Souls that remain in the Arcane for too long may end up as Demons.

The soul in itself does not interact much with anything except whoever has it, and thus, the most interesting stuff can be found in the sections about spirits and demons below.

Spirits
A spirit is a soul that has failed to "cross over" to the afterlife when a person dies, and thus, remain in the mortal world. Spirits are sometimes also known as ghosts, poltergeists, phantoms, spectres, and/or wraiths, depending much upon the location and nature of the spirit. Depending on how a person dies, their soul may fail to cross over to the supposed afterlife. Usually, this is due to the death being particularly violent and/or traumatic; historical examples include being tortured to death, being killed due to betrayal, loss of loved one, and suicide. It is theorized that the nature of their death traumatizes the dying in a way which prevents the soul to act as it normally should.

A spirit that remains in the world of the living may manifest in different ways. Previously mentioned names for spirits are sometimes used to describe the manifestation: A poltergeist is a violent and active spirit that causes mayhem and sometimes even physical damage, whereas a spectre or shade is a spirit who appears intangible and distant. In general, a spirit appears as how the person was while alive, is transparent to some degree, and cannot be touched physically. Certain spirits may be able to vocalize sound, mostly speech, or manipulate objects, like the aformentioned poltergeist.

Contrary to what some mages may claim, Nulls do actually possess a soul. However, because they are completely cut off from the Arcane, their souls are unable to cross over when they die. Unless they die a traumatic death that would create a regular spirit, a natural or otherwise "normal" death creates a spirit known as a wanderer. They differ from normal spirits in that they don't usually make their presence known, and are thus difficult to detect even by the most sensitive mage. Wanderers also, as the name suggest, "wander" about the world, and rarely stay for long in the location of their death.

Wanderers may become visible to the naked eye during certain "high energy" events such as lightning storms, large fires, or when a mage overexerts themself enough to become possessed by a demon (see below). Also, when Caroline makes a pact with her demon, their combined power in the field was enough to almost completely manifest all the wanderer spirits of fallen Nulls on the battlefield.

While wanderers are always harmless and do not act upon the material world, they could still be a jarring thing to witness for the uninitiated.

A spirit may be manipulated with the use of magic. This was taken to its logical extreme when Caroline caught on to the spirit of Svein, and merged it with a metal puppet, effectively reviving him.

Demons
Demons are, just like spirits, another form of the soul. Most knowledge that is known about Demons were gathered by the self sacrifice of a mage named "Edelward the Old" during the 13th century. By intentionally getting himself possessed by a High Demon, he was able to extract information from it and write it down for future generations.

Feral Demons
The natural state of a soul is to inhabit some kind of host - a human. When a person dies, their soul will more often than not go to the Arcane, where it will float around until it is reborn in a new body. A soul that, for some reason, is left in the Arcane for too long will begin to deteriorate and fall apart. The last stage of this process, which takes several hundred years, is a Feral demon.

Feral demons have completely lost all knowledge of what it means to be human. They are sentient, but no longer sapient, and will operate according to instinct and not logic. The most powerful instinct that drives a feral demon is its self-preservation drive, making them hostile to any other entity. This is not a problem in the Arcane, but if a feral demon finds itself possessing a human (which is, in the case of a feral demon, always by accident) it may pose a real danger to the mage. Such a possession has mostly gone under the name "Mage lunacy" in Druthenia, and can be "cured" with the correct method.

A feral demon that exists for long enough may eventually become a high demon.

High Demon
A high demon is the next step in the evolution of a demon. As feral demons are basically blank slates, something is eventually bound to fill it up. A high demon is a feral demon that has, over the span of hundreds or even thousands of years, created its own "humanity". In short, a high demon is a sort of corrupted mimicry of a human soul, that works much differently.

Being once again sapient and intelligent, high demons pose a much larger danger than any feral demon. They will actively seek out mages to possess, and when inside their mind, will slowly but surely break them down psychologically until the high demon completely takes control of the mage. While feral demons work on instinct, high demons are their own individuals, and will have different motives for gaining control of a host body; however, in the beginning it is usually about returning to the natural state of a soul.

A mage may find themselves possessed when they've been using magic that you'd normally find exceeds what is considered healthy. The great use of magic opens up the mages' connection to the arcane, and to the demons inside, basically makes them stand out like a lighthouse in the dark. High demons can take this opportunity to possess the mage, whereas feral demons can by mistake be "swept away" by the current of magic energy, accidentally ending up possessing a mage.

Posession
The process of possession is a long and complicated process, and will not be the same for every case. However, there are certain parameters that are consistent across the board.

A mage that is completely possessed by a high demon will no longer be reachable. Their soul still exists inside their bodies, but it lays dormant and silent, and lacks any control. A hostile high demon may also prove to be extra dangerous, as they can utilise not only their host's magic powers, but also their own: high demons are known to be incredibly powerful magic users. Thus, possessed mages are known to be the strongest and most dangerous mages known to the world.

Daema
A Daema is the name of a mage that has managed to curtail and take control of their demonic possession. Tales of such feats are rare and treated mostly as myths, until Quicksilver became a Daema during the end of the Northern war, when she realised that she could look back into the demon and read its mind herself.