A soul, a creation of God, is the eternal spiritual life essence of a person or being that persists and remains after its host, the body, dies or is destroyed. Demons, are beings of pure spirit who are corrupted souls and who just use human bodies as vessels to exist in corporeal form.
The soul is the source of a being's thoughts, memories, emotions, sentience, and consciousness. It was first created by God, when he created man, so that after death the souls could either be rewarded or punished depending on the life they chose to lead. Eve, the Mother of All, created monsters whose souls function similarly, but are destined instead to another dimension.
In the Natural Order, human souls are transported to the afterlife by Death's servants, Reapers; however, the reaper Tessa refused to tell Dean on more than one occasion where she takes them. Known destinations of souls are Heaven, Hell and Purgatory. However, since Metatron closed Heaven, the human souls who were supposed to go there are now stuck in the Veil, waiting for its reopening.
Usually, souls leave their vessels when they die, being taken to their final place. But they can be removed by force, leaving their owner's soulless. However, it seems souls have a link to their vessels, when removed, the souls return to them (if they are still alive) when they have the chance to do it.
Souls are magical by nature and could be used by several creatures to cast spells or to increase their power.
It was revealed in Sacrifice that angels who have been stripped of their grace gain souls and undergo the same process of life, death and afterlife, not unlike human souls. The soul, separate from its body, is visualized as a blinding white ball of light. Both Death and Famine own a black case that can house human souls, securing them until they can manage the contained souls.
Souls are said to be unbreakable but not permanent or immune to harm. Death claimed that not even he could break a soul. According to Henry Winchester, a damaged soul could heal in due time.
Although the fate of ghosts was at first unknown, it was revealed that the souls of the ghosts are taken to their final destination by Reapers after their remains are burned, rather than going into oblivion as was previously thought.
The body can be made uninhabitable for a soul by use of a spell, that requires the person to commit patricide. According to Balthazar who suggested this to Sam, "You need the blood of your father, but your father needn't be blood." One could kill not just their father but also a father figure for the vessel to become scarred.
Crossroad Demons were the first to show the use of souls. In exchange for a soul they would grant humans any desire they wish. Once the exchange was completed, the demon would give the person ten years before they came to collect. It can be as little as one year ( Dean's deal) to as long as twenty-five years (Becky's possible deal). Either way the demon has to collect and send the soul to Hell. Once in Hell, the soul is tortured until it has become so twisted and damaged, it is no longer human but a demon. Due to the fact that the demon starts out as a twisted soul, burning its original human bones will kill a demon similar to putting a ghost to rest.
Death and Fairies know whether someone has a soul by just looking at the individual, along with the Alpha Vampire. However, angels must first perform a painful procedure where the angel reaches into the human's stomach (Bobby called it a "soulonoscopy"), indicating that souls are tangible. Death being able to physically handle souls also substantiates this premise. Archangels like Michael and Lucifer can abuse them in their true form without using a vessel.Both angels and demons can use souls to gain more power. Castiel used Bobby Singer's soul to heal himself; he also used the power of 50,000 souls to blast Raphael, effectively starting the civil war in Heaven. When Castiel opened a portal to Purgatory using a ritual, he absorbed the souls of every monster there and the power he gained from them was enough to easily trap and kill Raphael. The souls from Purgatory also made Castiel immune to the deadly effects of an Angelic Blade. With his own power drastically augmented, Castiel claimed the title of God.
Apparently, angels can absorb a soul's experiences or memories, absorbing them into their allotted vessel, as Castiel did for Sam.
Eve, the Mother of All, also referred to the souls in Purgatory as her 'supply', implying she could also draw power from souls.
Effects on HumansEdit
For humans, naturally, the soul gives them feelings and a conscience. When Castiel rescued Sam from Lucifer's Cage, his soul remained trapped in the Cage until Death retrieved it for him. Without his soul, he continued to have physical needs such as food, drink and sex. He also doesn't sleep, had a high tolerance for pain, and was completely emotionless. At one point, Dean refers to him as being a robot. It can be said a person without a soul is similar to a sociopath.
Souls can also affect a person mentally. Because of Sam's soul being in the Cage for so long with Lucifer and Michael, Castiel tells Dean it could make Sam a drooling mess. In order for Sam to survive his soul's return, Death erects "a wall" in Sam's mind to help him survive. When Castiel destroyed this wall, Sam is plagued with memories of his time in Hell and is nearly driven to insanity by a hallucination of Lucifer. Supposedly the spiritual scarring also leaves mutational damage upon the body; the Leviathan Sam, who recovered some of Sam's DNA, also complained of the psychological issue, suggesting that the soul's damage can transfer into the DNA as well, as opposed to being confined to Sam's soul and consciousness.