Shadows are a person's id. They are the part of one's personality that is suppressed as unacceptable: whether because it's personally repellent, or because it's detrimental to their role in society, or because it's a part of themselves that simply runs counter to societal rules. Often, this part of the personality is buried so deep that people remain unaware of what they have rejected and locked away. For more detailed discussion, see Personal Shadows.
While it is not considered IC information, Personas and Shadows are two sides of the same coin. A Persona is a controlled Shadow, and a Shadow is an uncontrolled Persona. When an individual loses control of their Persona, it reverts into a Shadow: a thing of primal desires, unconstrained emotions, and runaway passions, unchecked by the higher and more rational desires of the user. Should the individual then surrender control to this Shadow, it will gain dominance. The result is the condition known as 'Shadow-Possession.'
Yielding to one's Shadow is essentially giving in to one's base urges. The clarity of simply giving in affords the Shadow-Possessed power beyond that of most Persona Users (as detailed in Upgrades). However, this is at the price of loss of control, and the deterioration of a healthy and developed sense of self. If left unchecked, the Shadow will gradually erode the user's sense of self completely, until they are no longer human but a mere wandering Shadow.
Being Shadow-Possessed does not mean one is 'bad' or 'evil.' Most often, yielding to one's Shadow is simply the result of person with a weak self-image manifesting a Persona and being unable to control it. There are a number of reasons why a person might have such a weak self-image to begin with:
The most common method to become Shadow-Possessed is to become so repressed and beaten down from extended psychological trauma that the basic personality is simply unable to bear the strain anymore. Not just any normal stress would cause Shadow-Possession, however. It generally requires the active malevolent influence of others to cause a breakdown of the main personality without harming the Shadow too much to be useful.
Those who fall into Inaba's Midnight Channel must eventually face their own Shadow. If they immediately accept and yield to this Shadow, the unopposed Shadow will gain dominance and possess them. Shortcuts to power do not exist; simply accepting one's Shadow wholesale does not grant the understanding and self-control necessary to maintain and control a Persona properly.
Abuse of Power
When one uses supernatural power, one must always be careful. To abuse it too much for one's own selfish gratification-- especially when one lacks the strength of will and concrete self-identity to keep themselves from spiraling out of control-- may hasten a Persona's reversion into a Shadow.
Someone who is Shadow-Possessed does not necessarily immediately stand out as such. Usually, there are subtle changes to the personality of the Possessed. They are less restrained and more likely to take immediate, impulsive actions. Not all of these actions are even necessarily evil, but they can often be inappropriate or disruptive.
Instead of summoning their Shadow, as a Persona-user might summon their Persona, a Shadow-Possessed literally transforms into it. When a Shadow takes full control of the Possessed, they undergo a dramatic physical change, drawing shadows to themselves and melding them into a new body. This form reflects the nature of the Shadow: someone with the Shadow Fafnir, for example, would literally assume the form of a dragon.
In general, a Shadow-Possessed possesses the same suite of capabilities as a Persona-User, expressed through whatever monstrous shape they wear. This shape-- constructed of loose 'Shadow material'-- is unliving and malleable, and freakish things can be done with Shadowforms within the limits of its basic size and form. It's an all-or-nothing transformation, however-- there is no such thing as partial transformation-- and no powers are accessible while untransformed.
Shadow-Possessed also share a number of traits with the wandering Shadows of the Dark Hour and the Midnight Channel. A Possessed has a flanged voice and yellow eyes (similar to Persona 4 TV Shadows) when in Shadowform. They're also perceived as 'just another Shadow,' even while in an untransformed state, and will generally be either ignored or left alone.
Transforming also gives a Shadow-Possessed a limited ability to command weak Shadows around them (e.g. Mayas from Persona 3). However, being 'just another Shadow' has a downside; should the Possessed encounter a powerful Shadow like an Arcana Shadow or TV Shadow, there is a chance they may wind up controlled by it if they are not strong enough to resist.
Shadow-Possessed, like regular Persona-users, may summon additional generic Personas. However, since Shadow-Possessed do not see the Velvet Room (unless they are trying to recover from Possession), and cannot access it without an invitation from a Persona-user, Shadow-Possessed must gain generics by eating the wandering Shadows around them, thereby tapping the collective unconsciousness. This urge to consume other Shadows is an instinct while in Shadowform.
When a generic is activated, a Shadow-Possessed's main Shadowform mutates in appearance to incorporate visual aspects of the generic. For example, a person whose main Shadowform is a lion, who subsequently activates the generic Persona Harpy, may wind up with a Sphinx-like form as a net result.
Shadow-Possession is not a static state. The condition can wax and wane; the more an individual embraces his/her Possession, the more monstrous they will become, and vice versa. The stronger the will of one who is Shadow-Possessed, the more stable the possession is. A rare few can even seem to almost control their Possession completely.
However, in most cases, over time the Shadow will gain greater and greater sway over the person until they become completely dominated. This process can be hastened or even immediately caused by eating too many Shadows, or attempting to consume one very powerful Shadow. At this point, the Shadow consumes the original personality, dissolving the individual into a wandering Shadow. The character is functionally dead at this stage, as there is no known method of restoring someone from this state.
Ultimately, to recover and transform the Shadow back into a Persona, the Shadow-Possessed must integrate and accept their Shadow as a healthy part of themselves, rather than continue to allow it to retain sole dominance.
It is almost impossible for a Shadow-Possessed to do this on their own. In their state, they cannot marshal the effort and power needed to regain full control. It requires a great deal of psychological support over time to rebuild the self-image to the point where one might find a way to confront their Shadow once more on relatively even terms.
One means by which a Shadow-Possessed may be 'cured' of their condition is possessed by Department 4. The Special Handling Unit is a machine capable of a sort of 'surgery' upon the psyche, repairing the psychological damage that causes the Shadow-Possessed condition. For more information, refer to the SHU file.