Throughout the Doctor's adventures, he accumulated much knowledge and experience that was codified in his biodata. During the War in Heaven, much of that information, such as the biodata codes of assorted Time Lord presidents, was lost. Thus, when the Doctor died, his body became very valuable to many Wartime powers including the Time Lords, their enemy, Faction Paradox, the Celestis, UNISYC, the Daleks, and the Krotons.
Most psychics retained some power even after death, depending on their powers while alive; the Doctor's psychic potential was so significant that the Relic still generated a psychic "call", particularly attuned to nearby humans, for long after the Doctor's death.
The Relic was contained in a metal box with two Greek letters on it. The metal glowed, though not in a way that lit up the space around it; it only stopped glowing when the Eighth Doctor threw the last bit of dirt on it on Quiescia. (PROSE: Alien Bodies)
After the Doctor's death, the Relic was planted in a bunker under the ruins of the city centre of Smithmanstown. There, it was found by the Corporation. Sharp-toothed Celestis agents worked with Little Brother Kolman to steal it for Faction Paradox, but Cousin Sanjira didn't recognise its importance and instructed Little Sister Justine to dematerialise it. From there, it crashed onto Earth and was put in the Toy Store, which was pillaged when the Daleks invaded. By 2169, the Black Man had it in his store in London, though he sold it to Qixotl shortly before Homunculette came looking for it.
Qixotl set up an auction for the Relic on Earth in the late 21st century, creating the Unthinkable City to hold it. He invited only a select group of people interested in the Doctor's body for an actual purpose, rather than risking inviting those who just wanted to mutilate it for revenge against him. The auction did not go to plan, due to such factors as the Krotons taking the place of the Daleks and the inherent danger of the various powers bidding for the Relic.
The Eighth Doctor arrived after hearing news of the auction and soon determined what the Relic actually was, even if he avoided learning any specific details about where or how he would die in the future. During the subsequent crisis, at one point the Doctor managed to link with the Relic's psychic energy to expel a Faction Paradox agent that had possessed one of the human delegates.
In order to take the Relic for himself, the Doctor convinced the Celestis that they had "marked" him as their agent in his eighth body in the present, rather than immediately prior to his death, making the Relic into a temporal paradox. In reality, the Celestis had marked Mr Shift, a conceptual entity that the Doctor had trapped in his own mind before he confronted them, leaving Shift to be driven insane by his new enforced loyalty to the Celestis and the Enemy. The Doctor was free to leave with the Relic, which he took to the planet Quiescia and destroyed with a thermosystron bomb. (PROSE: Alien Bodies)
Visiting a potential future in which he had died on Trenzalore, the Eleventh Doctor identified his now dying TARDIS as his tomb. Entering the TARDIS, he correctly anticipated that he would not find his corpse but rather his time stream. (TV: The Name of the Doctor)
Behind the scenes
- PROSE: The Book of the War notes that the future Doctor who becomes the Relic is a "particularly interventionist" 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 the Relic is the body of the "Infinity Doctor".