Edom is is one of many worlds—a mirror dimension to the world known, where history played out in a slightly different manner. It is also the realm of Asmodeus and Lilith, now considered a demon realm of Hell. According to some tomes found by Alec Lightwood, Edom had a capital—a demon city known as Idumea.
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. To counter them, the Nephilim were created by the Angel Raziel, gifting the first of them, Jonathan Shadowhunter, with angelic blood and Idris.
The Nephilim of this world were different, however: they lacked the Silent Brothers, Iron Sisters, and, thus, weren't able to develop 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—the demons. In their last days, 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, the Shadowhunters eventually defeated and wiped out.
The asmodei burned away the sun of the world and filled the sky with ash; they burned the world down, drained the seas, and killed every living being, leaving only the lesser demons to scavenge for food in the barren, burnt-out wasteland.
In 2007, Sebastian hid in Edom, where he was untraceable from his own dimension. He 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, the Greater Demon agreed to send them back to their world in exchange for a sacrifice.