In the early 1800s, Inquisitor Dearborn interrogated Annabel Blackthorn in order to obtain Malcolm Fade's location, as he had stolen the Black Volume of the Dead. Because he believed that her relationship with the warlock tainted her enough to render the Mortal Sword useless on her, he tortured her for information.
When it became clear that she truly didn't know anything, Dearborn had her sent back to her family—tortured and half-mad—to say goodbye before being sent to the Adamant Citadel. When the Iron Sisters sent word that she never arrived, Inquisitor Dearborn called in Felix Blackthorn for questioning regarding her whereabouts. From a statement made by a younger Blackthorn accompanying Felix, he suspected that Annabel was killed by the family and proposed a trial by the Mortal Sword for Felix.
When the younger Blackthorn questioned why Dearborn cared so much, considering the condition Annabel was returned in, Dearborn brushed it off, saying she was no more hurt than any Shadowhunter may be in battle, but told them her death is worse than the torture she had endured from him, and that the Iron Sisters were concerned. Dearborn ultimately ended the search for Annabel after one of Felix's Herondale friends pointed out how well the illusion of her being an Iron Sister benefited the Shadowhunters, particularly in regards to their standing with Malcolm Fade, and was backed by Felix's other friend, recognizing that Felix intentionally brought his friends to avoid punishment.
Memories of these incidences were recorded in an aletheia crystal, which were later retrieved and watched by Kit Herondale, Ty and Livia Blackthorn in 2012 with the help of a warlock called Shade who was familiar with the Inquisitor.
Dearborn was a corrupt Inquisitor. He showed no regard for justice as he was willing to ignore Annabel's murder by her family, despite it being a major crime, if it benefited him and Shadowhunters as a whole. He also showed the typical disgust towards Downworlders as other Shadowhunters of that time and was quite cruel, as he tortured Annabel and showed no remorse for it.
Inquisitor Dearborn was a slightly young man with dark hair.
- Lord of Shadows (first and only appearance) (appeared in projected memories)