Abystrum

Abystrum, or the Deep Realm, is a shadowy realm to which demons are banished. It is dark and cold, and is a twisted facsimile of some regions of the Overworld. Its existence was first elucidated by Falladin, who called Abystrum "a realm of darkness, chaos, and eventual entropy".

Nature of Abystrum
Abystrum is a realm that serves as a prison for demons. Luxius used his holy light magic and an unknown mechanism to repulse demons, pushing them into Abystrum. Abystrum is imperceptiply dark and frigid. Its atmosphere is inhospitable to the human body, and any human in a physical form would be choked by its toxicity. The realm itself is a motley collection of derelict buildings and entire swaths of land banished to the abyss, of which more intelligent demons flit in and out. The bottom of Abystrum is pure solid blackness, with lesser demons fighting each other endlessly.

No human alive has witnessed Abystrum in person; much of its depiction is a combination of speculation combined with cryptic information relayed by the gods. What is known is that the realm is designed to be a prison for demons; it is often a one-way trip to enter this realm.

Abystrum's Location
Since it is another realm, it is impossible to access Abystrum through normal physical conventions in the overworld. It is speculated Luxius used some sort of device to safely put human souls in Abystrum, while they were initially setting up the prison sometime around 100 Po.D.

After the Coalescence, an unknown substance matching the description of the bottom of Abystrum appeared in abundance. Tainting the Overworld with its presence, it is a common belief that Abystrum was "merged" into the Overworld and primarily exists in the Wastes. Upon meeting Iza, it was revealed to the party that Abystrum is below the castle, slowly leaking demons into the world above.