At some point, Horace's wife died, and he was left to raise Zara to share his hateful views on Downworlders and follow in his footsteps. After the Cold Peace was established, Horace gathered a group of radical Shadowhunters who shared his views and together started the Cohort, establishing himself as the unofficial leader.
In 2012, Horace kept track of his daughter's progress at the Los Angeles Institute by trading letters with her. The pair intended to take over the Institute, with the ulterior motive of pushing his proposal for the Registry further. He later attended the Council meeting where Annabel Blackthorn was to testify and spoke out against her and the Blackthorns, taking part in the heckling of Annabel by the Cohort members.
Following the Inquisitor Robert Lightwood's death in the hands of Annabel, Dearborn was appointed as his successor. Shortly after his appointment, and after the funeral rites for Livia Blackthorn and Robert, Dearborn made plans to interrogate all Downworlders and part-Downworlders in Alicante. When Jia Penhallow found out, she warned Magnus Bane and the Blackthorns to give them the opportunity to leave and instead asked Julian Blackthorn and Emma Carstairs to remain for interrogation for their involvement with Annabel, and the destruction of the Mortal Sword, to appease Dearborn.
Horace is a brown-haired man with an expressionless face.