Purgatory is a supernatural dimension created by God to contain the most dangerous beasts, Over time, it became the destination of the souls of monsters and the worst people out there.
Purgatory may be adjacent to hell, but implied it was quite difficult to access directly. Only inhabitants who find a way to escape though, it's rare they knew how to locate it and therefore, create a passage to it. Purgatory functions as a prison and the final realm of souls. A spell can be used to get to Purgatory through a lunar eclipse and blood from one of Purgatory's natives. This spell could also be reversed in order to return the souls back into Purgatory.
Conversely, beings other than monsters may also find themselves transported to Purgatory; the Bone Of Righteous Mortal Washed In the Three Bloods of Fallen is shown and implied to send anyone near the person it's used goes directly to Purgatory along with the Leviathan. When something other than a monster enters Purgatory, it seems that monsters that are there can sense the foreign beings, as word seemed to travel fast that a human and an angel had been sighted in Purgatory.
Because Purgatory was not designed to hold humans, there is a portal located in Purgatory which will become active when humans are near it, and will send any humans who enters it back to Earth in the Hundred-Mile Wilderness in Maine; it was indicated that the portal was created by God as an "escape hatch" in the event that any humans became trapped in Purgatory.
There are estimated to be 30-40 million souls in Purgatory. In Slade Strikes Back, These multitude of souls were sought by Slade the leader of Slade's Ensemble and M.O.D.A.B's 2nd in command Bender, who both agreed to work together and use the power of the souls for their mutual goal to Stop The Joker, while Bender and Slade did it for their own reasons like Bender to stop Joker's outright aclopaystic plans and Slade to elminate The Joker from conquering his empire and stopping others to do so. The pair attempted to locate Purgatory by way of capturing and torturing different monsters, whch Slade considered to be False Disney Stars.. This attracted the attention of a very angry mother of False Disney Stars, who was at first content with the natural order, but was convinced that she needed to protect her children and her realm. She began to arrange for her children to create armies of their own and to summon her to Earth.where she planned to turn all human souls into monsters in order to increase the power of her realm and to deplete Slade of future resources. However, the struggle resulted into her defeat through Dib.
Slade and Bender both eventually find the door, through the use of the horsemen of the Apocalypse and the rings that keep them from using their powers which they win in a multiuniversal auction where The Joker also attended so he can enslave them to start the apocalypse while Slade and Bender use them to try to prevent the apocalypse. Bender wins the auction and Joker tries to kill both of them so he can get it by default. Slade and Bender summon them and use a binding spell to have them reap souls so the two can find Purgatory. They do and then steal a great deal of the souls which they use to stop Joker, and Slade uses his half to maintain his power and then sends them back afterwards know that he found a way to replicate the power.
Ironically The Joker is in Purgatory and many of the worst villains are there.
There's a few rules a wanderer from Earth must follow: for example, one must never eat/drink anything nor accept temptations there; it was named the Maze of Temptation for a reason, otherwise a soul is bound there for eternity. This was the case when Ichabod was willing to free Abbie after exchanging her soul with Katrina's. Also, under Moloch's reign, in order for someone to leave Purgatory freely, one must be willing to sacrifice his/her own soul in exchange for a prisoner's. There was an area within Sleepy Hollow that allowed access to this world, and there was also a relic known as the Gehenna Key that's made to allow Moloch to escape on Earth, however, the plan was foiled by the two Witnesses, as it was proof that objects used in Purgatory crumble to dust when they leave. Katrina, Abbie, Moloch (the Demon King), and a fallen angel named Orion were residents there.
During the story, The Greenhorn's Gamble, Ichabod visits Purgatory to rescue Abbie's soul from Moloch, and Gavin dives in after him. While there, he finds a buried blade and names it the Purgatory Sword which easily slices away the shades, excepting Ichabod's doppelganger and Moloch himself. They prevent Abbie from drinking the offered beverage and lead her away from Moloch and his army before they got a chance to escape. The Gehenna Key, and an incantation, helped the trio and the portal closed just seconds before Moloch broke through.
In the Jossed fic, A Crane's Bane, Ichabod's parents had formerly resided there: Vladimir and Celeste Crane, whih the latter was imprisoned by the former after she was murdered by him. But, after an exorcism spell done by her and Ichabod, Vlad went to Hell aftrr his escape to Earth during the great earthquake. While Celeste eventually got her spirit lifted thanks to a ritual done by the Onondagas.
In Gavin's Purgatory Journey, he was put into a dream state by Onondaga shamans at their reservation in Basin Falls. Unlike Sam's predicament, there would be no Leviathans escaping. He ventures up Mt. Purgatory and Katrina's used as his spiritual guide. Along the way, he repents for each sin on every terrace as an angel removes the seven P's engraved on his forehead (thru the Ante-Purgatory, Pride, Envy, Sloth, Wrath, Avarice, Gluttony, Lust). He soon meets his mom, Lucy, at the Lust terrace and they both purify their souls as they dive thru a wall of flame. Gavin declares for his dad, Don, who was corrupt by sex from the succubus, Sallie, replace her when his time ends. He then meets Henry Parrish at the summit leading to the Earthly Paradise who performs the ritual to eat away his sins and no longer be bound to Don.
There was also a monster dwelling there known as the Leviathans created by God; it was within Purgatory that'd contain this said monster until they broke free and inflicted control over Castiel as well as others. That was until their leader, Dick Roman, got his head blown off.