Changeling

A changeling is a demon inhabiting the body of a human.

Possession

A demon can become a changeling by possessing a human. Many demons see this as a way of escaping their inconsistent existences in oneirosea and becoming "real".

When a human dreams, their soul drifts into oneirosea, but remains connected to their body by a spiritual tether. A demon who finds a dreaming soul in oneirosea can swim through this tether into the body and occupy it, taking the place of the original soul.

Incomplete Phase

When a demon first takes possession of a human body, it becomes an incomplete changeling. During this phase, the demon is merely a visitor temporarily occupying the body. It can physically control the body, but retains all of its demon powers and can voluntarily leave by swimming through the tether back into oneirosea.

Incomplete changelings are depicted with the concentric circle eyes of their demon forms. However, it seems that this is a stylistic visual effect only visible to the audience. In-universe, changelings appear identical to their hosts. As long as they don't behave suspiciously or use their powers, they can only be distinguished through mist-sense.

During this phase, hadal- and benthic-rank changelings are so powerful that their presence may physically reshape their host bodies to more closely resemble their demon forms. Some benthic-rank demons are too powerful for human bodies to contain at all, and warp their vessels until they lose all structure and eject the demon into reality. This invariably results in the death of both the original human and the demon.

Complete Phase

As time goes on, the demon's presence in the body erodes the tether away. If it maintains possession of the body until the tether snaps, the demon permanently bonds with the body and becomes a complete changeling. The amount of time this process takes varies from case to case, with the average being 6 hours. Some humans' tethers are more durable than others, and it seems that stronger demons erode tethers faster than weak ones.

Once this phase is reached, the demon takes the place of the original soul and can no longer leave. The changeling becomes indistinguishable from a normal human to all known tests, biological and oneirological. All demon-specific powers are lost, though complete changelings tend to have higher mist capacities than average and can re-awaken similar human powers. Over time, possession of a human brain makes changelings psychologically more human-like as well. Memories of their life as a demon are retained, but often seem unreal or "like a dream".

After the tether snaps, the original human soul is trapped in oneirosea forever. In most cases, it dissolves into mist and disappears. Very rarely, the soul has enough willpower to keep itself together and becomes a demon of its own.

Exorcism

Defeating a changeling and banishing the demon back to oneirosea is known as exorcism. This is one of the main duties of Alcyon Corporation's defense division, but independent exorcists unaffiliated with Alcyon Corporation also exist.

Changelings can be exorcised by knocking the body unconscious. Any method that would render a human unconscious works, including exhaustion, concussions, or chemical methods. However, any damage inflicted by an exorcist will carry over to the body's proper owner, so care and precision are necessary. Standard procedure for Alcyon Corporation agents is to immobilize the changeling, then administer a spray-based soporific drug developed by the company.

Once exorcised, the demon is forcibly sent back to oneirosea through the tether. At the same time, the human soul stops dreaming and returns to the body through the same path in the other direction.

Exorcism is only possible during the incomplete phase. Once the complete phase is reached, the demon permanently bonds with the body it possesses, and can no longer be separated. As such, exorcists must be alerted to the possession, reach the changeling, and perform the exorcism, all before the time limit is reached. This means independent exorcists who do not have access to broad information networks and lots of manpower are very limited in effectiveness.

Aftereffects

Even if a human is successfully exorcised, the aftermath of possession can have lasting effects. These effects become more severe the longer a possession lasts, so time is of the essence for exorcists.

Almost all victims of possession experience sleep deprivation symptoms, though in most cases they are mild and can be treated simply by getting some rest. In more severe cases, or those who have survived multiple possessions, the brain can be affected. Memory loss and confusion have been recorded.

For those who had their bodies warped by the possession of a high-rank demon, the changes do not revert after the demon is exorcised. Surgery is necessary to remove them, and in some cases even this is not sufficient.

Victims of possession often have their mist capacities increased by the experience. This makes them capable of developing powers, but the increase in mist also makes them more appealing targets for repeated possessions.

Lucid Changelings

This section contains major spoilers for the Oneirosea series.

Lucid changelings are a rare variant of complete changeling, caused when an incomplete changeling is exorcised at the very end of the time limit. The human soul's tether snaps while the demon and human soul are both travelling through it, and a mix of the two souls ends up inhabiting the body together.

The effects of this vary based on how much of each soul ends up in the body:

• If not enough of either soul remains in the body, the changeling is afflicted with mild to severe brain damage symptoms, such as amnesia, aphasia, or coma. Some manage to regenerate enough soul matter to recover and live as normal people, while others waste away and die.
• If the majority of one soul and a small portion of the other is retained, the changeling may experience memory loss or personality shifts, but otherwise the symptoms are not severe. If the demon dominates, it can use this to its advantage and pretend to be the original human more effectively.
• If the majority of both souls remain in the body, they merge in unpredictable ways. The changeling essentially becomes a new person with elements of the original human and demon, with parts of the memories and personalities of both.

Alcyon Corporation's R&D department discovered the existence of lucid changelings more than 11 years ago. However, it was decided that this information would not be released to the public. This is because it would cause mass panic and paranoia, which would lead to stressful dreams that are easier for demons to infiltrate, making reality more dangerous overall. As such, very few people know of lucid changelings, most of them being Alcyon Corporation's senior researchers or command division members.

Those with this affliction are simply told that they are experiencing heavier aftereffects than usual, due to the length of time they were possessed. This seems like a natural extension of the fact that longer possession causes heavier aftereffects, and thus is rarely questioned.

Trivia

• Diel shells were developed to emulate the function of the human body in an incomplete changeling, but without progressing to the complete phase.