Edom is one of many worlds; in this case, it is a mirror dimension to the world in which we all know, where history played out in a slightly different manner. Edom is believed to be the dimension to which Lilith was banished. Eventually, the asmodei, the demon children of Asmodeus, came into the world and threatened its peace. Just like in the other world, the Nephilim were created in Edom by the Angel Raziel. The dimension even has their own version of Idris, complete with most of the country's elements.
The Nephilim of this world were different, however: they lacked the Silent Brothers, Iron Sisters, and seraph blades, putting them at more of a disadvantage against the demons in which they fought. The Nephilim here were also more prideful, and ultimately refused the help of Downworlders, which caused the warlocks and faeries to align themselves with their infernal parents—demons. While the Shadowhunters of this realm had no seraph blades, they created a weapon called the skeptron to ward off the demons invading their world. The weapon was fairly effective, but even still, the world was eventually overrun by demons and has since become a barren, burnt-out wasteland, where lesser demons scavenge for food.
In 2007, Sebastian hid in Edom, where he could not be tracked by the Nephilim from their known dimension, and used the dimension's version of Alicante's Gard as his base of operations, with the intention of someday ruling the realm with his sister. Clary, Jace, Alec, Isabelle, and Simon traveled to the realm through a connected pathway in Faerie in search of Sebastian and his hostages: Luke, Jocelyn, Magnus, and Raphael.
In Sebastian's fortress inside the Dark Gard, he was finally confronted and killed, but not before Sebastian, thinking Clary had agreed to become his queen, had sealed all of the passageways between Edom and their home dimension, essentially trapping her and her friends in Edom. When Magnus finally called on Asmodeus, his father, for help, Asmodeus agreed to reopen a door for them to get back to their world, in exchange for Simon's immortality and his memories of the Shadow World, knowing the suffering of Magnus and his friends as a result would be even more of a payment for him.
- Based on its description, it can be inferred that this world was something of a parallel dimension to the world within the Shadowhunters universe.