It is described as the deepest, most heavily protected part of Hell. It currently holds the archangels Lucifer and Michael, as well as Michael's substitute vessel Adam Milligan. Sam Winchester also spent some time in the Cage, but was "rescued" by Castiel. Unfortunately, Castiel was unable to raise Sam with his soul. Sam's soul spent well over a year (in Earth time) in the Cage being tortured by Lucifer (and possibly Michael too) until Death finally retrieved it.
The interior of the Cage, according to Castiel, is heavily guarded, more so than when Dean was in Hell, suggesting the existence of other entities (possibly angelic, maybe Seraphs, guards) in the Cage or could simply mean that the Cage may be labyrinth-based. During his time in the Cage, Sam is seen surrounded by flames (conforming to Death's description of it as a "fiery pit" - or Death may just have been referring to Hell in general). This Cage seems to have multiple entrances. Unlike most angels, fairies including the Leprechaun, may be able to enter the Cage undetected, as they have a way of getting through "back doors" of prisons, though, just revealed, Castiel finally admitted that he was the one who raised Sam. Despite the complexity of the Cage, Death travels through the Cage unaided, successfully retrieves Sam's soul, returns to Earth, and places it inside Sam.
The fact that Lucifer's Cage can be opened from almost any point on Earth, (as Dean opened it in a house in Detroit and later on Sam opened it at Stull Cemetery) implies that it has an existence in another dimension or a separate plane of existence, as implied by Dean's description of time passing differently in Hell.
Lucifer was once God's most beloved child. However, when God created Humanity and ordered his angels to bow down before them, Lucifer refused. Lucifer argued that Humanity was grievously flawed and used this to justify his actions, though in reality he was merely jealous that Humanity had replaced him in the foremost of God's affections. Lucifer decided to mock God by twisting a human soul Lilith into the first demon. For this unforgivable offense, God created the Cage - a prison cell within the deepest part of Hell. He then ordered his strongest, eldest angels and Lucifer's older brother, the Archangel Michael, to cast Lucifer into the Cage for all eternity. Lucifer pleaded with Michael to take his side instead, but Michael obeyed his father's command and cast his brother into the Cage.
The door of the Cage, originally located where Ilchester, Maryland would be founded millennia later, was then locked with a series of Seals, known as the 66 Seals, designed to prevent Lucifer from escaping Hell unchecked. Though there were well over 600 Seals, only 66 of these needed to be broken for the cage to open and Lucifer to be released, though the first and last Seals would be specific. The first Seal, which had to precede any others for them to be effective, would be broken when a righteous man would spill blood in Hell, meaning that as he would break, so would it break. Meanwhile, the first demon, Lilith, would be the last Seal, and as she would be broken, so would it. God also secretly created a set of master keys that - when combined - could open and close the Cage at their owner's will. These keys were the four personal rings of the Four Horsemen of the Apocalypse.
Though none of the demons knew the location of Lucifer's prison, a church known as St. Mary's Convent would come to be built right over the very spot which the door of the Cage was originally located. Eventually, Azazel would find it, and from here, would speak to Lucifer for the first time in many years.
Season 4Season 4 finale, "Lucifer Rising". In the opening of the episode, which takes viewers back to the year 1972, the Yellow-Eyed Demon, Azazel, is revealed to have escaped Hell and to have discovered the truth of St. Mary's Convent's status as the location of Lucifer's Cage after having wandered for years looking for it. He travels to Ilchester, Maryland and takes possession of the head priest, disemboweling eight nuns shortly thereafter during a mass in the convent's main chapel. Draping the head nun's bloody corpse over the altar, Azazel beseeches Lucifer to "whisper through the door" of the cage. Azazel successfully makes contact with the imprisoned Devil, who commends Azazel's faith in him and his ability to locate the Cage's entrance. Lucifer reveals to Azazel that Lilith, who is at this time locked away in Hell, is needed to break the Seals and set him free. It is here that the plan involving the Special Children is conceived by Lucifer and Azazel, with the aim being that the special child would be the one to ultimately kill Lilith to ensure Lucifer's release and then to serve as his vessel to lead the army of Hell against God in the Apocalypse that would soon follow.
Several decades later in 2007, Azazel orchestrated the release of Lilith through the Devil's Gate in Wyoming just moments before his own death, though this ensured that his plan to release Lucifer was now set into motion. In the present year of 2009, Dean Winchester has long since broken the first Seal during his time in Hell before his rescue and 64 of the other Seals have by this time been broken, leaving the final Seal the only thing standing in the way of Lucifer's release. Sam Winchester, unaware of Lilith's status as the final Seal on Lucifer's Cage, eventually follows the first Demon to the now-abandoned St. Mary's Convent in Ilchester, Maryland. Ruby has by this time gotten Sam hooked on Demon Blood, which enhances his demonic powers and secretly prepares his body to hold the Devil's essence, and accompanies him to the chapel to confront Lilith. Meanwhile, Dean learns during his sequestering by the Angels that Lilith is the final Seal and that her death will ensure Lucifer's release. Castiel, troubled by his conscience about what will happen to Humanity once the Apocalypse begins, disobeys his orders and accompanies Dean to the home of the Prophet Chuck Shurley to learn where Sam has gone. Though they learn the truth about St. Mary's Convent from Chuck, the Archangel Raphael begins to descend upon the house and Castiel remains behind to hold him off while zapping Dean to St. Mary's to stop Sam from killing Lilith.
Dean arrives just as Sam pins Lilith to the altar in the convent's chapel, where years before Azazel had committed the bloody massacre of the nuns. Ruby turns and smiles at him before raising her arms and locking the chapel's doors, locking Dean out. Though Dean attempts to break into the chapel and screams for Sam to stop what he is doing, Lilith's taunts are enough to push Sam over the edge and use his powers to kill her, unknowingly breaking the final Seal on Lucifer's Cage beneath their feet. As Lilith's blood pours from her mouth and forms a spiral circle sigil in the center of the chapel, Ruby reveals that the first Demon, Lilith, was the last Seal, and that she and Lilith were working together all along to release Lucifer from his cage. She tries to convince Sam that Lucifer will repay him in ways he cannot imagine, moments before Dean finally busts through the chapel's doors and kills the demon with her own knife. Sam apologizes to Dean, but the two brothers turn and watch in horror as the circle of blood begins to turn, revealing the mouth of Lucifer's Cage and opening it, allowing the Devil to rise out of Hell and ascend to the Earth. Moments before the Devil fully escapes his prison, God teleports the Winchester brothers out of the chapel and onto a plane passing over Ilchester at that moment, from which they witness Lucifer's angelic essence shoot up into the sky from the spot where St. Mary's Convent had stood before being blown apart by the Devil's escape from his cage, heralding the beginning of the Apocalypse.
The archangel Gabriel reveals posthumously, before being killed by Lucifer, that Lucifer's Cage, which was opened when the 66 Seals were broken, is still down in Hell and can still be used to contain Lucifer if it can be reopened and Lucifer tricked back into it somehow. He reveals, believing that Lucifer is not aware of it, that the original keys to the cage are the rings worn by the Four Horsemen of the Apocalypse. He suggests that if the Winchesters can gain all four, they will have the cage as an answer to stopping the Apocalypse from devastating the Earth and destroying Humanity. However, the Archangel reveals that this will not be easy as they must first get the cage door back open and then devise some way of tricking his brother back into it before shutting it again.Pestilence and Death, the last two of the Four Horsemen of the Apocalypse holding the other two keys to Lucifer's Cage. After successfully acquiring Pestilence's ring by force and learning of the Horseman's plot involving the Croatoan Virus, Sam goes with Bobby to stop the virus' outbreak while Dean goes with Crowley to Chicago, Illinois to acquire Death's ring. In Chicago, Dean has a civil conversation with Death, who openly despises Lucifer and the fact that the Archangel is merely using him as a weapon by means of a binding spell just to throw a tantrum and display his displeasure to his Father, God.
Death, who has no real interest in the Apocalypse and sees both it and its players as insignificant in the grand scheme of the Universe, agrees to willingly hand over his ring to Dean in exchange for Dean's promise that he will do absolutely everything necessary to put the Devil back in his cage and thus free him of Lucifer's service, even if this means Sam losing his life to do so. Death also describes Lucifer's Cage as a "fiery pit". The Horseman warns Dean to hold true to his word as he can't "cheat Death" and gives Dean the instructions on how to work the rings, which combine to form a key-like device that controls the opening and closing of Lucifer's Cage.Season 5 finale, "Swan Song", where Dean, out of options, decides to go along with Sam's plan that, as Lucifer's true vessel, he will consent to possession by Lucifer and fight him for control long enough to throw himself into Lucifer's Cage, taking the Devil with him. The group tracks Lucifer to Detroit, Michigan, the very place Lucifer had told Dean in one possible future that they would meet and Lucifer would acquire Sam as his vessel. Sam drinks an enormous amount of Demon Blood in order to prime himself to contain Lucifer's essence and says his goodbyes to everyone before going with Dean to confront the Devil. His current vessel heavily deteriorated, Lucifer hears out Sam's proposition to give his consent for possession, Sam proving his seriousness by effortlessly killing Lucifer's demon body guards with his demonic powers. Lucifer reveals however that he knows about their plan to trick him back into his cage and that the rings control it. Despite this, Lucifer likes the idea of an internal battle and gives Sam a proposition: if Sam can ultimately overpower Lucifer for supremacy, he may jump into the hole leading to Lucifer's Cage and take the Archangel with him; but if Lucifer is able to overpower Sam, then he wins all. Sam consents, and the Devil quickly abandons his vessel to become one with Sam, knocking both of them out momentarily. This gives Dean the chance to open the cage using the Horsemen's rings. Sam recovers and walks toward the portal before turning and smiling, revealing that Lucifer has overpowered Sam and is now in control of his body. He closes the cage, takes the rings, and vanishes.
Lucifer travels to Stull Cemetery in Lawrence, Kansas to meet his brother Michael for their final showdown the following day. Before the two can act however, Dean arrives in the Impala and demands to speak to Sam. Though Michael (using Adam Milligan as his vessel) protests his presence, Castiel hurls a canister of burning Holy Oil at him, causing him to vanish for several minutes and giving Dean time to try to reach Sam. Lucifer, outraged at Castiel's audacity, snaps his fingers and obliterates Castiel before telekinetically snapping Bobby Singer's neck after he attempted to shoot the Devil, who was brutally beating Dean on the hood of the Impala.
The Archangel tells Dean between blows that he will break every one of Dean's bones and make Sam feel it. Dean does not resist however and merely tells Sam that he will not leave him. As Lucifer goes to land another blow, he glances into the back seat of the Impala, where he glimpses an old army figure stuck in the ashtray that was put there by Sam in his childhood. Suddenly the Archangel is overwhelmed by all of Sam's happy memories of his life with Dean. Sam's love for his brother stimulated by this rush of memories is sufficient to break Lucifer's grip on Sam's body, allowing Sam to finally take control of the Devil at last. Sam tells Dean that everything will be all right and that he now has Lucifer. He tosses the Horsemen's rings to the ground and recites the incantation that activates them, opening Lucifer's Cage in the heart of the cemetery. Just as he is about to throw himself into the pit, Michael returns and demands a fight to kill Lucifer, stating that it is his destiny to do so. Sam ignores this however and outstretches his arms, beginning to lean toward the pit. Though Michael tries to stop him, he only ends up being pulled down into the pit alongside his brother as the cage shuts once again in a thunderous flash of white light, leaving behind only the red-hot rings of the Horsemen. After being healed by a restored Castiel, Dean goes to see Lisa Braeden and sits down to dinner with she and her son while outside, a streetlight flickers and there standing beneath it is Sam, free of Lucifer's Cage somehow.
Sam starts remembering a portion of events that happened when he lacked a soul. After Sam and Dean resolve the murders, Sam falls onto the floor, shaking uncontrollably, where he recalls some of the torture he endured in Hell by Lucifer, where he is seen covered in flames.
A portion of Sam's time in the Cage is briefly alluded to, though not really seen.