The Timewyrm is an evil goddess. You can tell how edgy she is by that "y", just like Vampyres are edgier than Vampires. She was the main antagonist of the first four novels of the Virgin New Adventures, following on from the end of the cancelled TV series.
Originally, the Timewyrm was an ancient legend from the ancient days of ancient Gallifrey who didn't really exist but still scared the pants off Rassilon and friends because ohmigod, what if she did exist.
Then some evil queen named Qataka was so evil that her evil minions rebelled and cast her out, so she became the Babylonian goddess Ishtar. A bunch of crap happened involving 13-year-old prostitutes, which you really don't want to read, and in the end, the Doctor threw her into the Time Vortex to defeat her.
But she was so evil that instead of being destroyed, she became The Timewyrm, and then ohmigod, she did exist. For some reason, she went to Earth again, because it's not like there are any other planets in the universe, and decided to take over Hitler's mind. At the same time, the mutated War Chief was also busy trying to use the Nazis to change history, because why not. The Doctor became good friends with Hitler and saved him, and everyone lived happily ever after. Seriously, that's what happens, and it's all written in that jolly Uncle Terry Target novelisation style.
This time, the Timewyrm was too weak to do anything but travel down the Doctor's own timeline, because that makes sense. So she rewrote a past Second Doctor story, and the Seventh Doctor had to figure out wtf happened, which he failed at miserably, but the Timewyrm was hoisted by her own Petard Machine.
So the Timewyrm went back to Earth again, because there still aren't any other planets in the universe. She moved a church to the Moon and then put the whole story inside the Doctor's mind, so the Doctor had to pilot the TARDIS into his own imagination. Bravo Cornell. Also, there are owls. The Doctor managed to de-evil the Timewyrm and turn her into a harmless baby. Obviously, Russell T. Davies never read this book (despite quoting from it in an episode of Confidential). And thus, after three false starts, were the Virgin New Adventures truly born, stories too wide and too deep for the small screen, and too far up their own asses to even be able to navel-gaze—when it works (like it does here) they can be pretty cool.
Cat's Cradle Edit
The Timewyrm quadrilogy was followed by the Cat's Cradle trilogy.