Possession is the state of a physical body being inhabited and controlled by an incorporeal spiritual entity to serve as a physical body for them. A possessed human body is called a vessel or (more colloquially) a meatsuit. Many spiritual entities and some higher beings gain corporeal form on Earth via possession, including demons, angels and others.
In the mortal realm, demons can possess human bodies as bodies for themselves to use to corporeally interact with the physical world. Unlike angels, demons do not need a human's consent to possess them, and they can possess corpses as well as living human bodies. Humans are especially vulnerable to possession when feeling scared or anxious. Demonic possession can be blocked by wearing anti-possession charms. Demons usually possess a vessel by entering the human body in their smoke form through the mouth. At least powerful demons such as Azazel can possess reapers as well.
Once a human is under demonic possession, the demon can take full control of the body from the human, and it can exhibit through the vessel its supernatural abilities and weaknesses; although the demon can also give the human back control if it wishes or the human can take back control from the demon through very strong willpower. When a human body under demonic possession, the demon causes the vessel's eyes to change colour (to black, red, white or another, depending on the demon's rank in Hell). If and when the demon is exorcised, the vessel will retain no memory from possession except for times when the demon let the human be conscious while the demon used their body. During demonic possession, if a living vessel suffers any injuries, then while the possession will stop these injuries from affecting the demon and keep the vessel alive, it will not heal or repair them; thus, if and when the demon leaves its vessel, then the injuries will take harm and effect on the now-disinhabited host.
Demonic possession apparently also causes a chemical change in the vessel's circulatory system, making the vessel's blood become demon blood; and whenever a demon uses their powers, they leave behind a trace of sulphuric yellow powder. It would appear that the amount of sulphur a demon produces depends on its power-level; most demons only leave small traces of sulphur whereas Azazel left piles of it. The biological changes that appear in vessels during demonic possession is said to be because when a body is possessed by a demon, the demon infects every cell, muscle and body part.
Exorcism and expulsionEdit
Demons can leave their human vessel by exiting through the mouth, which is called 'smoking out' due to the smoke form demons take on Earth outside host bodies; demons can either smoke out by will, or can be expelled from their vessel by means such as exorcism. However, angels can use their abilities to grab the smoke forms of demons smoking out and force them back into their vessels, and saying exorcism rites in reverse can also send smoking out demons back into their vessels; and there are some spells that can lock demons inside their vessels and render them unable to smoke out and/or teleport, as can some sigils carved onto the demon's host body. It has also been indicated that gagging or otherwise obstructing a demon's mouth can stop them exiting their vessel as well.
Among the means of forcefully removing demons from their vessels, the most common is exorcism. Exorcism is painful for the demon, and once the exorcism rites are completed, the demon will be expelled from its vessel and sent back to Hell. It seems to vary between demon exorcisms whether or not the vessel survives the process, although if the vessel suffered any fatal injury during demonic possession, then death is effectively certain. However, very powerful demons such as Abaddon, a Knight of Hell, are immune to exorcism. If a demon's vessel ingests large quantities of salt, this can also force the demon out of the vessel body.
Through training, tutoring and development and the consumption of demon blood to boost their powers, special children can develop their abilities to affect and harm demons; they can torture demons, pull them out of their vessels and send them back to Hell, and even kill demons when powerful enough. Ghosts can pull demons out of their vessels as well and physically grab and restrain demons in their smoke form.
Angels can possess living humans' bodies as vessels which act as corporeal forms for said angels that allow them to interact better and more conveniently on the physical plane; but unlike demons, all angels need a vessel to consent to possession before they can possess them (Season 5) - but the vessel does not need to directly say yes to angelic possession for the angel to possess them, but can also say yes to another thing which will involve the angel possessing the vessel to achieve it. (I Think I'm Gonna Like It Here) Angels can also possess graceless angels and share the graceless angel's vessel.
Though angels can possess most humans, unlike with demons, angels can only possess humans whose bodies are strong enough to house and contain said angels. If an angel possesses a vessel that isn't strong enough for the angel's power-level, then the vessel will gradually wear and burn away from containing the angel until it combusts, (Season 5, I Think I'm Gonna Like It Here, I'm No Angel) and the angel's power-channelling capabilities in the vessel will be more limited than in a stronger vessel.
The stronger an angel is, the fewer vessels strong enough to permanently contain it there are available. For Michael and Lucifer, only the Winchester family's bloodline stretching back to Cain produces vessels strong enough to permanently contain one of them, and even so, they each have only one true vessel out of that entire bloodline whom they can channel their powers to their fullest through. (The Song Remains the Same)
When an angel is possessing a vessel, the former can exhibit superhuman strength, stamina and powerful angelic abilities through the vessel, and it can sustain and repair any physical damage done to the vessel with earthly weapons and other weapons incapable of harming angels. Angelic possession is apparently sometimes a very unpleasant experience for the vessel; Jimmy Novak described being possessed by Castiel as like being bound to a comet, (The Rapture) and it has been implied that each and every time an angel uses its powers in its vessel and/or the vessel's possessed body sustains physical damage and is repaired by the angel, the vessel will feel this. (The Rapture, Like a Virgin) Angels seem to usually keep their vessel's consciousness suppressed and trapped within them, although an angel can converse non-physically with their vessel while possessing the vessel's body if they wish - Lucifer described doing this as "tak[ing] the gag off" (Swan Song) - and can also voluntarily give the vessel control of their body back. (Season 9) Though the angel's celestial attributes and influence are almost entirely gone from the vessel after the angel has left it, a small leftover trace of the angel's grace will remain within the vessel, but will fade away over time. (First Born)
Exorcism and expulsionEdit
Angels can leave their vessels whenever they wish, (Season 5, Season 9) or can be exorcised/removed from their vessels by higher and more powerful angels (The Rapture) or by an angelic-exorcism chant. (On the Head of a Pin) Also, the person an angel is possessing as their vessel can, through incredibly strong emotions and willpower, take control of their body back from the angel; (Swan Song) and if the vessel does have control over their body instead of the angel possessing them, then they can either trap the angel's essence inside them (Swan Song) or expel the angel from their body. (Season 9) It is also known that if a vessel's suppressed mind becomes aware that they're under angelic possession, then then may be able to expel the angel from their body by revoking the angel's permission to inhabit them (Season 9) - however, it would appear that this isn't case with archangels such as Lucifer. (Swan Song) After an angel has left its vessel, the vessel will be left alive but with little or no memory from when they were under possession, (The Devil You Know, Season 9) and the vessel may also feel very hungry (as angels do not need food and thus do not eat often in vessels). (The Rapture)
Archangels can render their vessels hollowed out and empty after angelic possession, leaving the vessel blank and catatonic after the archangel leaves it. (Free to Be You and Me, The Song Remains the Same) However, Michael indicated that the catatonic state in archangels' former vessels is caused by a lack of care on the archangel's part and the catatonia in the vessel can be voluntarily avoided; Michael once temporarily possessed John Winchester without any long-term ill-effects on John, and he claimed he would also not leave Dean catatonic had he possessed Dean for the Apocalypse. (The Song Remains the Same)
On Earth, Leviathans take mobile human form by possessing a being such as a human (through physical contact or ingestion by the human in question). After a human is possessed by a Leviathan, the Leviathan in question seems to fully and permanently possess their body and mind, also biologically altering the whole host body: the Leviathan's vessel becomes resistant to and able to repair and reassemble from almost all forms of damage; it bleeds black Leviathan ooze in place of blood; and when the Leviathan is about to attack or feed, its human form's face will transform into a giant mouth with sharp fangs and a bifurcated tongue for killing and devouring. In human form on Earth, Leviathans can also morph their human vessel to copy any other human, becoming completely identical to whatever human form in question (excluding the Leviathan attributes that Leviathan's human form exhibits). (Season 7)
Leviathan can also possess angels on the earthly plane, and more than one Leviathan can inhabit and control the angel's vessel, although if there is a large number of Leviathans then the angel will quickly explode and release the Leviathans from the pressure of containing the creatures in one vessel. (Meet the New Boss, Hello, Cruel World)
Eve was able to rise from Purgatory and manifest on Earth via a ritual through possessing a human virgin as a corporeal human form. In the ritual, the virgin girl to be possessed was to be sacrificed to Purgatory, and Eve would subsequently rise to Earth in the sacrificed virgin's physical form. (Like a Virgin) Similarly to the Leviathans, Eve seemed to fully take over and alter her host body; the virgin girl that Eve possessed as her vessel would bleed a watery black liquid, (Mommy Dearest) and even if and after Eve was killed, her vessel would be left braindead and yet still produce monster eggs. (The Man Who Would Be King)
Some ghosts are capable of possessing living humans, which causes the vessel to excrete ectoplasm, similarly to with demons; the human's consciousness is awake and can see what the spirit is doing with their body, but is locked away in their mind while the ghost controls their body. Salt can be used to expel ghosts possessing living humans, in that causing the possessed person to ingest enough salt can hurt the ghost enough to force the spirit out of the host person's body. (After School Special, Southern Comfort)
Though reapers are normally neutral and incorporeal, they have been known to be able to enter and possess humans' bodies and use them as vessels with which to physically interact with the corporeal world, similarly to with demons and angels. When possessing humans, reapers can also use telekinesis to move beings and objects with their minds. (Taxi Driver, I'm No Angel)
The first known Khan Worm was uniquely capable of monster possession of humans, in that it could enter a human's head through the ear and control their body and actions. The Khan Worm could still animate and control a host body even if it was dead. When someone was possessed by the Worm, they would exude a black-green slime through their ears and nose, especially when the Khan Worm was in pain. The Khan Worm could be killed and/or forced out of a host through intense electrocution of the vessel. (...And Then There Were None)
When they come to settlements, farms and villages they bless to collect their two annual sacrifices, a Vanir takes corporeal form by possessing and animating an effigy (such as a statue or scarecrow) as a vessel with which to hunt and kill its two sacrifices. After taking its two sacrifices, the Vanir will disinhabit its effigy vessel until the next annual cycle. (Scarecrow)