De Quincey had been the head of the vampire clan in London for many years. During this time, he became close and seemingly infatuated with Camille Belcourt, who had a werewolf lover named Ralf Scott. Since it was forbidden, at least in their culture, for their kind to love their mortal rivals, they saw each other secretly but were eventually found out by de Quincey. For this, de Quincey murdered him, thinking of it as a favor for his long-time friend.
In the 1870s, de Quincey was believed to be "the Magister" and leader of the Pandemonium Club. In reality, he had merely become disobedient of the real Magister. De Quincey wanted to be the one with the prestigious title, so the real Magister saw it fit to punish him by using him as a patsy until the first phase of his plan was completed.
In 1878, Camille reported to the Clave that Alexei has been killing mundanes at his parties as a way to celebrate flouting the Accords set by the Nephilim and breaking the Law. Camille herself revealed that her motivations for sharing this information to the Clave were so she can exact revenge on de Quincey for killing Ralf.
Will Herondale and Tessa Gray, who had Changed into Camille, attended his party with Magnus Bane in order to witness the alleged crimes. De Quincey greeted Tessa (in Camille's form) cordially and asked if he could have a taste of her human subjugate, Will, though Tessa politely refused.
De Quincey invited Tessa, Will and Magnus to the special event of the evening, which Camille had told the Enclave about. Tessa was horrified when she realized that the human about to have his blood drained was her brother, Nate. Will tried to signal to the waiting Shadowhunters using one of Henry's inventions, the Phosphor, and while the Shadowhunters were indeed alerted and soon arrived, the curtains and the house were set on fire because the device malfunctioned.
A fight broke out between the vampires and the Shadowhunters. De Quincey attacked Tessa, thinking she was Camille, telling her that he will never forget her betrayal. Will engaged him in battle and bit him in the process. Tessa attempted to shoot him in the heart but missed, and de Quincey, vowing revenge, fled through the window.
During Nate's stay at the London Institute, Nate began to put his master's plan into action by telling them that de Quincey was indeed the Magister. He further worried the group when he informed them of the Magister's plan to use a binding charm to animate his automatons, which the Dark Sisters will be able to accomplish during the full moon—that very same evening. Nate told them of his hideout in Chelsea, where he suspected he had hundreds of other vampires.
Charlotte, Henry and other adult Shadowhunters of the Enclave rushed off to the house, only to find him hiding out and recuperating with no more than seven vampires. When they accused him of being the Magister, de Quincey laughed and mocked them for being fooled by Mortmain. He was subsequently slain by Benedict Lightwood.
De Quincey had fair hair that was nearly white and fell straight to his shoulders. Typical of a vampire, de Quincey had very pale skin.
- Vampires, Scones, and Edmund Herondale
- Clockwork Angel (first appearance)
- Clockwork Prince (mentioned only)
- Clockwork Princess (mentioned only)
- City of Fallen Angels (mentioned only)
- He was partially named after writer Thomas De Quincey, whom Cassandra Clare has quoted a few times in The Infernal Devices.
- De Quincey commissioned a wallpaper design of hundreds of screaming humans on a blood-red background. The wallpaper was on display at his hideout in Chelsea when the London Enclave raided on the night of his demise.