Reality Virus

The Reality Virus was a universe-ending mechanism created by the First after she reawakened and was disappointed by what the universe had become. The Virus wiped out whole sections of time and space, leaving only a few warped remnants of reality. This occurred during the Thirteenth Doctor's lifetime, and the Doctor managed to trap the Virus within a time loop. (GAME: The Edge of Time) However, as the time loop progressed, the Virus eventually escaped again and rewrote the Tenth Doctor's life. (GAME: The Edge of Reality)

Behind the scenes
The Virus eventually also infected Gallifrey during the Last Great Time War. (NOTVALID: Doctor Who Fortnite)