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In the late 15th century, Diego was placed in charge of leading a bureau in Northern Spain, particularly in Burgos, where most of the Spanish Assassins including Aguilar de Nerha were unfamiliar with.
After the fall of Granada and the recovering of the Apple of Eden by Aguilar in 1492, the Mentor attempted to contact Diego for leads on the Templar Gustavo Ramírez's whereabouts, whom the Assassins believed to be in northern Spain. However, Diego was captured by Ramírez, who 'tortured' him for information on another section of the Staff of Eden. Ramírez was able to convince Diego to support the Inquisition and work as a double-agent for the Templars instead.
This led Diego to reveal the whereabouts of the last section of the Staff, allowing Ramírez to continue his search. As the Assassins arrived, Diego maintained his façade as a member of the Brotherhood and directed Aguilar and the Assassins to Sahagún for leads on Ramírez' whereabouts.
Sometime later, Diego travelled to Sobroso Castle, where Ramírez had recovered the last section of the Staff, originally part of the golden staff of Saint James. The Assassins arrived too late, witnessing Diego's betrayal and Ramírez retrieving the piece. Before they could confront Diego, they were stopped by del Salto, one of Ramírez' lieutenants.