Arthur Blackthorn was the brother of Andrew Blackthorn. Up until his death, Arthur was the official head of the Los Angeles Institute, though because of his deteriorating mental health, it was in fact his young nephew Julian who was running the Institute for him.
Both Arthur and his older brother Andrew were originally from London. They were raised by parents who were obsessed with Ancient Greek and Latin works, resulting in a shared fascination for it. Arthur himself eventually grew obsessed with the classics.
During his first year at the Academy, Andrew and Arthur were among ten students, accompanied by one of their teachers, sent to the Faerie to rescue a female Shadowhunter child that had been taken. While he normally would not have been part of the mission, he was allowed to accompany his brother, as everyone knew the pair always fought side by side.
When Andrew fell in love with Nerissa, a lady of the Seelie Court, Arthur insisted on staying with him. While Andrew and Nerissa became lovers, her sister laid claim to Arthur. He was then tortured throughout his stay, with his brother unaware of what was being done to him. When Andrew found out the truth. Nerissa, worried that Andrew would go mad with grief and guilt, made new memories for them. She also set up their "escape" by releasing Arthur, who then attacked her to rescue Andrew, and Nerissa let them believe that she had been killed.
After around seven years, the brothers returned to their world to find that only a day had passed since the day they chose to remain in Faerie. They reported everything they believed happened to the Inquisitor.
Arthur eventually returned to the London Institute while Andrew went to Los Angeles. Though their relationship was no longer as it used to be due to their traumatizing experience with the faeries, the brothers stayed in contact and remained close. Arthur visited the family, particularly during the holidays, and he often sent the children Latin works as gifts.
Meanwhile, Arthur conducted research at London's museums, befriending mundane curators to gain access to private collections.
After the attack on the Los Angeles Institute and the capture and subsequent Turning of Andrew into an Endarkened by Sebastian Morgenstern, Arthur was contacted by the Consul, Jia Penhallow and offered the leadership of the Institute as well as the guardianship of his brother's children. Arthur did not immediately agree to this offer, presumably due to the suddenness of the news. A few days later, the London Institute was also attacked by Sebastian and his Endarkened warriors, though the attack was deemed unsuccessful; even still, Arthur was injured in the battle and needed a cane for support afterward.
On the day of his arrival in Idris, Arthur attended the Clave meeting in order to discuss their post-Dark War plans. When some members suggested that the Clave strip his niece Helen of her Marks, Jia asked him to speak, and he publicly accepted the responsibility of taking care of his brother's children as well as the Institute. His nephew Julian was dismayed, as he had briefly hoped that he would no longer desire to help, and thus the Clave would be forced to make Helen their guardian. Though Arthur tried to vouch and plead for the loyalty and integrity of both Helen and her brother Mark, believing that they are both good Shadowhunters despite their faerie blood, a sentence was still passed down by the Consul: Helen was exiled to Wrangel Island indefinitely to study the wards, and the Clave also voted to abandon the search and rescue of Mark from the possession of the Wild Hunt.
Before the family left for Los Angeles, the issue of Emma Carstairs and her bond with Andrew's children was broached, and Arthur agreed to take Emma in as well, even before she became Julian's parabatai—a notion his nephew appreciated. Helen also tried to talk her siblings into trusting Arthur, with whom she, along with Julian, wanted to believe in and entrust her other siblings.
When Arthur arrived, however, it became clear that he would not be of any help to the children. He was not particularly kind when he introduced himself to the children, especially to Ty, whom Arthur dismissed as weird and different, and he hardly interacted with them at all. Even worse, Julian soon discovered that Arthur had lost his mind, due to his traumatic experience in Faerie and the losses he suffered during the war, making him incapable of running the Institute and looking after the Blackthorns.
In order to keep his family together, Julian secretly ran the Institute in his stead. Arthur rarely left the attic, where he spent most of his time studying, lost in his own head. To keep up the pretense that Arthur was the head of the Institute, Julian had Malcolm Fade make or acquire medicine that made him lucid for a few moments, in case of instances when his presence would be needed. Meanwhile, Julian personally looked after him and his own siblings. Arthur would often mistake Julian for his father, Andrew, because of their resemblance and his own lunacy.
After years of taking the medicine from Malcolm, Arthur's condition only worsened. His moments of lucidity grew shorter and shorter, his headaches more severe, making hiding his condition more of a challenge for Julian. The children's tutor, Diana Wrayburn, was aware of the situation but safeguarded the secret for Julian and also helped him cover for him.
At one point, Arthur befriended the head of city's vampire clan, Anselm Nightshade. In 2012, the vampire was caught by Jules using dark magic for his business, and to cover up for Arthur's obliviousness to their investigations on the Guardian and workings with faeries, Julian told Inquisitor Lightwood that Arthur had been investigating Nightshade, who was promptly arrested.
When Malcolm was discovered to be the Guardian and was believed to have been killed, Arthur and Julian lost access to the medicine that made him lucid, a poor circumstance given the sudden arrival of a group of Centurions. Once, the Centurion Zara Dearborn, whose father had plans of taking control of the Institute, snuck into his office and discovered his madness. When Ty, Livvy, and Kit found out the Dearborns' plans, they told Arthur, who only laughed and exposed Julian's secret: that it was Julian who had been running the Institute.
When the Institute was attacked by Malcolm and his sea demons, Arthur, to everyone's surprise, came rushing out of the Institute and joined the battle. Arthur was shaken by Malcolm's threats and rushed inside the Institute, dragging Ty and Livvy in with him. When the Blackthorns opened a Portal to London, Arthur waited until everyone had gone through. When only Kit was left, he asked Kit to thank Julian for him and stayed behind, intent on surrendering himself to Malcolm.
He traveled to Malcolm's hideout and offered his life in exchange for his family's safety. Malcolm agreed and, after taking him to where he had been keeping Annabel's body, slit his throat and spilled his blood to awaken Annabel.
Arthur resembled his brother, albeit smaller and paler. He had a slight figure, a round face, brown hair, and the trademark Blackthorn blue-green eyes. He also wore spectacles.
Arthur was named after King Arthur.
- Pale Kings and Princes (flashback)
- City of Heavenly Fire (first appearance)
- A History of Notable Shadowhunters & Denizens of the Downworld
- Lady Midnight
- Lord of Shadows
- Arthur's favorite weapon was the dagger.
- Arthur had been known to say that Lady Nerissa, Helen and Mark's mother, is alive, even though she is known to be dead. This was possibly due to his mental illness, which caused him to see people who are not there.