The Demon Halls, also referred to in Kal'sea as Hell, is the physical location within which all demons reside, and which they are born. The Demonic Halls are a world of fire and brimstone, of rivers of blood and the sounds of torture and dying. It is unknown of the relation between the Demonic Halls and the other realms it might adjoin, but its relation to the Mortal Realm is mostly one way: demons can be brought from the Demonic Halls through pentagrams, but mortals can't be brought into the Demon Halls in the same way. However, it is known that some demons collect human souls to keep in the Demonic Halls, allegedly to torture for all of eternity.
The true nature of the Demonic Halls is mostly unknown, as very few have ever travelled there and lived to tell the tale. It seems as though there has been an active effort to contain things from ever breaching this realm, as the only permanent connection to the Demonic Halls, if there is one at all, are in the rumoured Hellgates that are attributed to Kal'sea. The Demonic Halls are supposedly endless, its skies blackened by smoke. The predominant theory for the Demonic Halls' existence is that when Soahc was banished from Engelain, he was sealed behind a door in the Halls of Janus, and there trapped with his own essence began to bleed into the void, growing and contorting until a realm was made of his rage, horror and disgust, hence the demigod's epithet of Forefather of Evil.
The Demonic Halls is supposedly split up into various continents, levels, circles, or some other dividing system whose meaning is unclear to us- perhaps the nature of the divide is applicable only to the physical laws of the Demonic Halls itself, and therefore non-understandable by us mortals that have never had to have lived in such a realm. Various demon kings and demon lords rule over these various distinct areas of Hell, plotting against one another, though some believe that they are ultimately outranked by a higher being.
There is some suggestion that the Demonic Halls and the Realm of Magic might be one in the same. There are also theories within demonological studies which state that demons within their own realm have a non-corporeal state, where their consciousness mingles into one collective super-consciousness, though the veracity of any of these claims is dubious.
When demons die in the mortal realm, they return to the Demonic Halls, and so it is assumed they are immortal- it is unknown what would happen if a demon were to die within its own realm, or if such a thing were even possible. Also, given the sheer scale of power that the average demon has, it is widely agreed upon that if a sustained portal between the realm of demons and that of man were to be opened, then an apocalypse would certainly follow.