According to the ancient tome Myths & Legends, spirits are powerful energy beings that exist in a plane beyond our own.
Every living being on the planet has a soul. The soul is eternal and lives beyond death. Naturally, upon a living organisms demise, the soul passes on to the next life, which is not yet known.
In its purest form, the soul is made up of energy. That energy can be captured upon a person's death and contained within an object--or "vessel"--using the Dark Arts. This process is known as "infusing" the soul. Infusing souls is a common activity among spirits.
When a soul is infused, it is unable to pass on to the next life. As a result, the being that possessed that soul in life becomes trapped within the environment in which he was killed. These trapped beings are, at times, able to interact with the living world, both visually and audibly (but never physically) in a way similar to those trapped inside the Lifestream.
These trapped beings are the main cause of reported "hauntings" of residences. The truth, however, is that these beings--or "ghosts"--are not haunting the environment; they are simply unable to leave it.
Their souls remain trapped until one of three things occurs:
In order to transfer the soul to a new vessel, the third party must find an object that symbolizes something that the trapped soul loved most in life. Love, being the most powerful emotion, can overpower the spell that binds souls to a particular object. The third party must then find the "essence" of the trapped soul--that is, the pure energy of the soul itself, most often found inside the spirit realm--and place the object that embodies the soul's love within that energy.
The soul will then infuse itself with that object, leaving its original vessel for a new one.
If the physical embodiment of the spirit that captured the soul dies, the soul will be released from its confinement and allowed to pass on to the next life.
Spirits are dichotomic: positive and negative energy. "Positive" spirits are commonly known as angels, while "negative" spirits are commonly known as "demons"--though neither is truly good nor evil, as these are human constructions. In their pure energy state, spirits are not visible in our world and cannot interact physically within it.
The plane in which spirits exist is known as the "spirit realm." It is possible for humans to enter the spirit realm. To do so, humans must first possess a tangible item from that realm, which can be anything from an ancient relic to a simple piece of paper. Possessing this item will allow a human to see the realm for a short duration; but in order to fully enter it, that human must voluntarily present the item as a gift to the spirit whose plane that human wishes to visit.
Many spirits attempt to lure unsuspecting humans into their realm by giving them an item and later convincing them to hand it back over to them without the human realizing what that action will cause. Once the human presents that item, he can be fully taken (willfully or not) into the spirit realm.
The spirit realm itself can take any shape or form the spirit within that realm desires. When a human enters into the spirit realm, his physical surroundings may change completely (suddenly appearing as a different environment), slightly (room lights change from off to on), or sometimes not at all! Trickier spirits know how to influence the human's surroundings to mask the fact that the human is actually within the spirit realm, in order to remain in control of that human.
The lifespan of a spirit is, by nature, eternal. Their physical form can, however, be killed.
Every spirit that takes a physical form or "host" has a physical origin of power--an object to which that spirit is bound. (Oftentimes, if an object in our world is legitimately labeled as "cursed," that object is simply the physical origin of a spirit.) If this object is destroyed, then so, too, is the host. If the object is only damaged, however, then that spirit's power remains but is greatly diminished. The only way for the spirit to then recover this lost power is to repair the damaged object.
A spirit's downfall is its dependency on humans, which is why spirits often lure humans into their realms. Whereas humans require food in order to survive, spirits receive their nourishment from strong human emotion, both positive and negative. When a spirit feeds upon a human, that spirit sucks the emotion out of him. The human is nearly always unaware of this process and will feel no reaction beyond fatigue as a result.
Spirits are prone to capturing a human's soul upon death and infusing it in a vessel. The more souls a particular spirit collects, the more powerful it becomes.
A spirit can only infuse a soul after its host body dies. When infused, the soul will remain in a state of unrest, unable to escape its vessel but also unable to die.
Angels take the souls of humans already on the verge of death, so as not to influence the balance of life and death in our world. While this may sound noble, know that it is common for angels to manipulate events in such a way to indirectly cause that death. Demons, however, are prone to taking souls of humans directly through murder. As stated earlier, spirits cannot interact physically in our world when in their pure energy state. Thus, spirits must make use of a host body through possession.
Possession occurs when a spirit takes over the body of a living host. During possession, the host himself is aware of events occurring around it but unable to influence those events. The host's body is under the direct control of the spirit and will not age.
Possessing a host is the only way in which spirits can tangibly influence our world. Thus, it is of great importance to spirits to make use of this ability. Both angels and demons are well-versed in the art of possession, and both are prone to using it to suit their own selfish needs.
In most instances, possession is a difficult process that takes time. To possess a host, a spirit must first lure that potential host into the spirit realm and keep him there for a prolonged time. This means that the host must willingly present the spirit with a gift.
Once the host is in the spirit realm, the spirit can and will slowly take him over. Possession can only occur for small lengths of time at first. The host may not be fully aware of these instances. Successful periods of possession increase the bond between host and spirit, allowing the spirit increasingly greater lengths of time to possess the host in the future.
During early attempts at possession, the host will experience a strong sense of vertigo and will see flashes of events from the past, from the perspective of the spirit who is possessing him.
While the host is powerless to resist possession while in the spirit realm, possession itself can only exist outside that realm once the spirit has fully attached onto the host. Thus, in order to avoid permanent possession, the host need only to leave the spirit realm before this occurs.
If a spirit manages to fully attach onto a host in the spirit realm, however, then that spirit is able to retain possession when the host leaves the spirit realm. If this happens, it is too late for the host to ever retain control of his body again, even outside the spirit realm.
A possessed host can act and sound exactly like the host himself prior to possession. This is because the spirit possessing the host has full access to the host's memories. There is no way to definitively determine whether or not a particular host is possessed, and there is also no way to free a possessed host from his captor, despite supposed (and ultimately ineffectual) "exorcisms" that are routinely performed.
Last, it is important to note that a spirit will live on if its host dies during possession.
While the spirit will have to find a new host should it wish to remain in our world, the spirit itself is not harmed in any way.
Long before recorded history, demon Claude Ferucil bound himself to a prism and used it to exist in our world, during which time he fed off the souls of unwitting humans while possessing artistic hosts. He was finally stopped by Brianna Auberon in December 2011.