Reality Virus

The Reality Virus was a highly sophisticated artificial plague which distorted the very nature of reality. It infected and rewrote the past, present and future. (GAME: The Edge of Reality) It 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 was eventually weaponised and upgraded by the Cybermen. This version of the virus rewrote the Tenth Doctor's life. The virus was destroyed, reasserting reality in the process, by a human companion. (GAME: The Edge of Reality)

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