Wracked with guilt and blaming himself, Arno sought out the Assassins, discovering their headquarters underneath the Sainte-Chapelle. He was then inducted into the Brotherhood and vowed to hunt down François' murderers to redeem himself for his failures.
After assassinating Sivert and Le Roi des Thunes, Arno learned from Marquis de Sade of a silversmith who crafted the poisoned pin that killed de la Serre, named François-Thomas Germain. After tracking down the silversmith, Germain claimed that man named Lafrenière commissioned it.
Arno tracked down and assassinated Lafrenière, only to learn that he was the writer of the letter to de la Serre, warning him of a potential betrayal. Arno reported to the Council, who were outraged that he assassinated Lafrenière without their consent. When Arno mentioned that he was planning on attacking a hotel, not an Assassin base, the Council sent him to investigate the meeting.
Infiltrating the hotel, Arno eavesdropped on a meeting of a group of Templars, led by an unknown figure plotting to eliminate Élise. Arno escaped the hotel and managed to save Élise from the Templar ambush. Afterwards, the pair returned to Germain's residence, only to find it empty, as the man had anticipated that his deception wouldn't hold up. Élise informed Arno that Germain was exiled from the Order for his radical views and heretical notions about Jacques de Molay. Following this, Arno brought a blindfolded Élise before the Assassin Council.