Asmodeus is a Greater Demon, particularly a Prince of Hell and a fallen angel. He is also the father of the warlock Magnus Bane who, because of Asmodeus' influence and power, is often let off by other demons.
Prince of Hell
During the sixteenth century, a seeress prophesized that his addiction to taking human memories and emotions would be his undoing, though he was never troubled by this.
Asmodeus supposedly has many children: he has demon children in Edom called asmodei, as well as warlock offspring on Earth. By 2007, most of his warlock children have supposedly died, presumably because he has already claimed them and their immortality, and lives, which Asmodeus uses to fuel his realm.
Magnus Bane is a notable offspring of his, through a woman whom he tricked into believing he was her husband. Magnus has lived for almost four centuries by 2007, and Asmodeus claimed that he has made him the most proud. Magnus once called on him to verify his identity, but never again for years.
The Dark War
Lilith's "son", Sebastian Morgenstern, used Edom as his and his army's base and retreat as it is undetectable by the Nephilim. Sebastian's followers acknowledged their power and often dedicated their massacres, presenting the souls or their captures as sacrifices, to Asmodeus and Lilith, the rulers of Edom.
When Sebastian sent his kidnapped Downworlder representatives to his realm, Asmodeus had control over Magnus' magic and will. With this, Asmodeus weakened Magnus in hopes of making him call on him for help, an urge which Magnus fought.
Before his death, Sebastian destroyed the pathways between Edom and the Shadowhunters' dimension. Desperate, Magnus called upon his father using an inverted version of the Lord's Prayer for his friends to ask him to send them back to their dimension. After some small talk, he demonstrated his power by transporting Luke, Jocelyn, and Jonathan's body back to Alicante.
Knowing what Asmodeus wanted, Magnus offered to trade his immortality for the safe return of his friends. However, Simon Lewis offered to give his own immortality instead of Magnus'. Pleased to receive his Daylighter power and his immortality, Asmodeus gladly accepted Simon's; though because of Simon's short life, in comparison to Magnus' hundreds of years, he also insisted on taking not only his immortality but also his memories of the Shadow World, as well as his friends and families', promising to give Simon his mundane life back instead of leaving him the undead corpse of a vampire he was.
Asmodeus appears as a shifting patch of shadow before evolving out of the darkness as a tall man with a human face, pale skin seemingly stretched and taut over bone, and what looks like a coronet of barbed wire on his head. He has sharp, pointed teeth and gold-green eyes that are slit-pupiled like a cat's, just like Magnus'. When he met with Magnus and his friends, he was dressed smartly in a pure white suit with silver cuff links and buttons carved in the shape of flies.
- Jace Herondale once mentioned that he was a general of Hell's army.
- In 1929, after Aldous Nix opened a Portal to Hell, a demon spoke to Magnus and claimed to be his father. He was uncertain about the demon's identity at the time, but Magnus eventually found out and must have contacted him some time after this.
- The Rise of the Hotel Dumort
- City of Lost Souls (first mention) (chapter 11)
- City of Heavenly Fire (first appearance) (chapter 23)
- Born to Endless Night (mentioned only)
- A History of Notable Shadowhunters & Denizens of the Downworld