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.

Mythology
Even before Qataka was named as such, the Timewyrm existed, and would always exist in some form, as a creature from the Green and Black Book of Gallifrey. It was envisioned as a dragon that circled the universe until it ate its tail, and a prophecy foresaw that at the end of time, when Fenric slipped its chains, the Timewyrm would consume Rassilon in its jaws. (PROSE: Timewyrm: Revelation)

Another Time Lord prophecy predicted that Gallifrey would survive attacks from numerous threats before its ultimate destruction, the Timewyrm proving to be among them. (PROSE: The Gallifrey Chronicles)

The Fourth Doctor learned of his role in the creation of the Timewyrm when he was appointed as Lord President and granted access to the Matrix, (TV: The Invasion of Time) but automatic safeguards erased his memory of his discovery, leaving him only long enough to leave a recorded message for his future before he forgot about the message. (PROSE: Timewyrm: Genesys)

As Qataka
The Timewyrm began as Qataka, a brilliant but amoral inhabitant of the planet Anu. Obsessively fearful of death, and aware of stories of the Time Lords, she sought immortality. She experimented with cybernetics and made several breakthroughs, then began stealing brain tissue. She linked her own brain to a computer to preserve her memories and found she could use this to dominate the minds of those in whom she implanted radio receivers. Qataka's crimes were discovered and she was captured, tried and executed. However, she had uploaded her mind into a cybernetic body built by her slaves. This resembled a giant snake with platinum alloy skin. She fled Anu with her slaves, destroying the planet with a cobalt bomb. (PROSE: Timewyrm: Genesys)

As Ishtar
Utnapishtim and his followers had already left Anu. He pursued Qataka to the Sol system and fought her. Qataka's slaves died and her ship was destroyed, but her escape pod landed on Earth, where she met Gilgamesh. She read his thoughts, took on the identity of Ishtar, the local goddess of love and war and tried unsuccessfully to mentally enslave him. She was later found by Dumuzi, the high priest of Ishtar in nearby Kish. She took control of his mind and was soon virtual ruler of the city-state.

Ishtar used her slaves to put copper plating around the walls of Kish. This would have turned the city into a giant radio transmitter and she would take control of every mind in the region before taking over Earth. Utnapishtim uploaded a computer virus into her, unaware she had programmed a cobalt bomb to detonate on her death, destroying the planet.

The Seventh Doctor disarmed the bomb by linking her computer systems to the telepathic circuits of his TARDIS. This let Ishtar upload her consciousness into the TARDIS and she tried to take control of the ship. The Doctor jettisoned Ishtar and a piece of his TARDIS into the Time Vortex, hoping to destroy her, but she survived in the Vortex. He had helped create the Timewyrm by allowing the fusion of the Time Lord technology with the mind of Qataka. (PROSE: Timewyrm: Genesys)

As the Timewyrm
The Timewyrm escaped Earth, which she decided to destroy to spite the Doctor. Looking for a suitable host, she chose Adolf Hitler, but did not count on his mind's strength and was trapped. The Doctor and Ace, having been to an alternate timeline in which Hitler used the power of the Timewyrm to win World War II, tracked the entity to Dunkirk and banished it from Hitler's mind, scattering it throughout time and space. (PROSE: Timewyrm: Exodus)

The greatly weakened Timewyrm decided to travel back into the Doctor's personal timeline and strike when he was at his weakest: arriving at a point just after his first regeneration, subsequently hiding in his mind. During the Second Doctor's visit to the Panjistri homeworld in the far future, she found a suitable host in Lilith and transferred herself into her. She used most of her power extending her host's lifespan the five thousand years it took the Panjistri to complete the God Machine, which she planned to use to conquer time and space. However, when Raphael merged with the God Machine, the resulting entity banished the Timewyrm. (PROSE: Timewyrm: Apocalypse)

The Timewyrm used Chad Boyle and Anthony Rupert Hemmings as agents in a struggle on 20 December 1992. It took place simultaneously on the Moon (where she had transported St Christopher's Church from Cheldon Bonniface) and the Doctor's own mind, the Timewyrm having planted a seed of itself in his subconscious during their initial confrontation. Aware of this, the Doctor had intended to crush the Timewyrm, but when Ace was trapped in his mind as well, the Doctor took the TARDIS into the real-world/imagination interface to confront it directly. At the end of the battle, he banished the Timewyrm's power into dormancy and erased its memories. Its 'essence', however, was implanted in the mind of a baby (who had no upper brain functions before the transfer, having been a genetically-engineered infant intended to be used for disease research in the future). He gave the baby to Emily and Peter Hutchings, who lived in Cheldon Bonniface and asked they name her Ishtar. (PROSE: Timewyrm: Revelation)

After this, with Bernice Summerfield and Ace, the Doctor fought the Timewyrm once more, when she had downloaded herself into puterspace and plotted to corrupt the timeline. (COMIC: The Last Word)

As Isthtar Hutchings
In 2010 Ishtar met Chris Cwej at Benny's wedding in Cheldon Bonniface, have a dalliance with him and bear his child, Jasmine Surprise Cwej-Hutchings. Her middle name, "Surprise", was the same as Benny's. The Doctor briefly reactivated Ishtar's powers as the Timewyrm to save his old friend, the Brigadier, when he was killed before his predicted time, Ishtar's temporal blast not only eliminating the Master's minions but restoring the Brigadier to life and youth. (PROSE: Happy Endings)

Behind the scenes

 * The Timewyrm's last appearance took place in a special Doctor Who Magazine comic strip entitled The Last Word, intended to celebrate the tenth anniversary debut of the Virgin 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 returned.