He was injured on Simia KK98 and encountered an incarnation of the Doctor. Following this, he was taken by the Enemy and changed from a liquid-based lifeform (as a Gabrielidean) to an idea-based lifeform and became known as the Shift. He/It attended the auction for the Relic on Earth with the aim of making sure none of those invited obtained it.
Although unable to speak itself, the Shift could communicate by modifying the perceptions of others- either individuals or a group- so that they could "see" its message in other ways, such as making it appear that words were visible in the pattern of the stonework in a wall, or that someone else was speaking for it by modifying another person's perceptions to hear the Shift's words rather than the actual speaker's.
The Eighth Doctor was able to trap the Shift in his mind and trick the Celestis into marking it as their agent when they attempted to mark him. As a result, not only did the Doctor obtain the Relic — the body of his future self — by tricking the other attendees into thinking that the Relic was now a temporal paradox as the Celestis had marked him now rather than immediately before his death, but the Shift was driven mad as it was now mentally torn between its new status as an agent of the Celestis and its original role as an agent of the Enemy. (PROSE: Alien Bodies)