The Doctor (Alien Bodies)

The final incarnation of the Doctor died during the War; his corpse became the Relic. (PROSE: Alien Bodies)

Biography
Throughout the Doctor's adventures, he accumulated much knowledge and experience that was codified in his biodata, including the biodata codes of assorted Time Lord presidents. (PROSE: Alien Bodies)

Two days between the first battle of the War on Dronid, one of the more interventionist and optimistic members of the Great Houses visited the planet, claiming he had an old connection to one of the enemy's agents there. However, his diplomatic attempt apparently failed, and he was thought to have died as the battle started. (PROSE: The Book of the War)

However, Qixotl was involved in the true story of the Doctor's death, which was much more complicated. Even after the fighting on Dronid had already begun, a dying soldier was visited by the Doctor on Simia KK98; at that time, the Doctor said that he was just observing the conflict, that he wasn't working for the High Council, and that he was only one of the enemy "depend[ing on] where you stand."

At some point, the Doctor travelled to Mictlan and made a deal with the Celestis. (PROSE: Alien Bodies)

Behind the scenes

 * PROSE: The Book of the War suggests that this future Doctor is a diplomat on the Homeworld. Given the suggestions in that story and PROSE: The Taking of Planet 5 that the War King is the Magistrate from PROSE: The Infinity Doctors, this lines up with the idea that this Doctor is the incarnation from the Infinity Doctors universe.