The Logos was a book that, according to George Tremayne, was created by a race on Earth aeons before humans existed. The Third Doctor stated that its creators were so afraid of it that they wrote themselves out of the universe.
The book could change the universe by linking with a mind. George let it link with him but it killed him and used his body to try to rewrite the universe. However, according to the Doctor, it was so old that it was full of errors and would cause space-time to collapse if it wasn't stopped. The Doctor, with Edward Grainger's help, stopped the Logos with a "self-destruct" glyph. (PROSE: Ancient Whispers)