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 enacted, Horace gathered a group of radical Shadowhunters who shared his views and together started the Cohort, establishing himself as its de facto leader. One of his goals was to have the Clave create a Registry of all Downworlders.
In September 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 immediately held an emergency meeting to convince members of the Clave that his successor must be named, and he was appointed as the new Inquisitor. 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.
He was quick to use his new position of power against their enemies and to push for the Cohort's plans. Urged on by Manuel Casales, they plotted to have the Unseelie Prince Oban replace the current King to more easily sway them towards their plans. He sent Manuel and Dane Larkspear to Faerie to see it through, then Julian and Emma afterwards to retrieve the Black Volume of the Dead from Annabel, who'd escaped to the Unseelie Court. Horace meant for Manuel's team to intercept Julian and Emma once they'd gotten hold of the book and ordered to have them killed. Things didn't go as planned, and Julian and Emma made it back. He was at least able to use Dane's death and presented his corpse to the Clave as proof of worsening faerie-Shadowhunter relations.
While Manuel was there, he and another faerie were under the impression that Clary Fairchild and Jace Herondale went through the Portal to Thule and were as good as dead. Manuel came to him with this news, and he once again used it to declare war on faeries, the deaths of the heroes convincing Shadowhunters around the world to take up arms against Downworlders. Dearborn called for all Shadowhunters to return to Alicante to witness his staged parley, as a show of power, with the newly-ascended King Oban of the Unseelie Court, having previously agreed on a set of terms: an alliance against Seelie and other Downworlders, and an agreement to live peacefully apart.
|“||Horace had failed to protect anyone while he was alive. Perhaps things would be different now that he was dead.||”|
–Manuel's thoughts about Dearborn as he hid under his corpse, Queen of Air and Darkness
The parley was interrupted by Livia's Watch, led by Julian and the others. Julian, armed with the new Mortal Sword, exposed his lies and plots to every Shadowhunter watching the parley. Though he tried to stop a battle from breaking out, or at least being the cause of one, his followers attacked when the faeries on the field began fighting. Horace hid behind his followers to avoid the battle. When Julian and Emma became gigantic true Nephilim, Julian spotted him on the field, picked him up and snapped his spine, killing him instantly. Manuel hid under his corpse as protection.
|“||Horace Dearborn was one of the type who flourished on unrest and fear.||”|
Horace was a blond-haired man with an expressionless, nondescript face. He was a big man, bony instead of muscular, with big shoulders and enormous hands. In 2012, his left arm was cut from the elbow down by Diana Wrayburn.