Timewyrm

Before reforming and living out her life as a Human, the ever-changing apocalyptic super-being known as the Timewyrm threatened the universe more than once.

As Qataka
The Timewyrm began as Qataka, a brilliant but amoral inhabitant of the planet Anu. Obessesively fearful of death, and having heard stories of the Time Lords, she sought to grant herself immortality. She experimented with cybernetics and made several breakthroughs, then began stealing brain tissue from others. She linked her own brain to a computer to preserve her memories, and found that she could use this to dominate the minds of those she had implanted with radio receivers. Qataka's crimes were discovered and she was captured, tried and executed. However, she had uploaded her mind into a cybernetic body constructed by her slaves, which resembled a giant snake with platinum alloy skin. She and her slaves fled Anu, destroying it with a cobalt bomb.

As Ishtar
Utnapishtim and his followers, having left Anu earlier, pursued her to the Sol system and battled her. Qataka's slaves all died and her ship was destroyed, but her escape pod landed on Earth in Sumeria. Shortly after arriving there, she took on the identity of the local goddess Ishtar, who represented sex and fertility. (NA: Timewyrm: Genesys)

As the Timewyrm
When the the Doctor linked Ishtar's computer systems to his TARDIS, Ishtar was able to influence it like a computer virus. The Doctor jettisoned Ishtar and a piece of his TARDIS into the time vortex; thus the Timewyrm was born. (NA: Timewyrm: Exodus)

The Timewyrm managed to escape the Doctor and his companion Ace to Earth and altered the ending of World War II to create an alternate time line in which the Nazis had won. In doing so, the Timewyrm became trapped inside the mind of Adolf Hitler. The Timewyrm spirited away the Britisch Freikorps Hemmings in a replica TARDIS. (NA: Timewyrm: Exodus)

The Timewyrm was then pursued to the planet Kirith in the far, far future. (NA: Timewyrm: Apocalypse). She escaped and used Hemmings and Chad Boyle in her agents in a struggle in the year 1992 which ranged from the small English town of Cheldon Bonniface to Earth's Moon to the Doctor's own mind. Finally, at the end of the battle the Timewyrm's power was banished into dormancy while its memories were erased. Its 'essence', however, was implanted inside the mind of a baby (who had no upper brain functions before the transfer). He gave the baby to Emily and Peter Hutchings, who lived in Cheldon Bonniface and asked that they name her Ishtar. (NA: Timewyrm: Revelation)

However, after this, alongside Benny and Ace, the Doctor fought the Timewyrm one more time, at which time she had downloaded herself into puterspace and plotted to corrupt the time. (DWM: The Final Chapter)

As Isthtar Hutchings
In 2010 Ishtar would meet Chris Cwej at Benny's wedding in Cheldon Bonniface, have a dalliance with him and bear his child. (NA: Happy Endings)

Behind the Scenes
The Timewyrm's last appearance took place in a special Doctor Who Magazine comic strip entitled The Final Chapter, intended to celebrate the tenth anniverary debut of the New Adventures series with Timewyrm: Genesys. Almost certainly, this story takes place before Benny's wedding, but it does not explain how the Timewyrm has managed to return, though fans can easily think of explanations for this.