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 Books 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) This knowledge existed at the core of the Matrix, the oldest input from Ancient Gallifrey, which the Fourth Doctor described as "very apocalyptic, end of the Universe" and the Seventh Doctor described as a horror unleashed upon the multiverse. (PROSE: Timewyrm: Genesys)

Another Time Lord prophecy predicted that Gallifrey would survive attacks from numerous threats, including the Timewyrm, before its ultimate destruction. (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)

Power
A unique creature, the Timewyrm was able to slip between times, able to go anywhere and any when in the universe and feed on minds. (PROSE: Timewyrm: Genesys) In addition to feeding on them, the Timewyrm could completely possess an individual and extend their life for thousands of years, (PROSE: Timewyrm: Apocalypse) although it could become trapped in the person's mind if they were strong enough. (PROSE: Timewyrm: Exodus)

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, combining her mind with the power of the secondary console room that he had ejected. 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 by convincing Hitler that he didn't need it, 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, the Timewyrm cast off its physical form and existed purely in Puterspace. Using Bernice Summerfield and Ace, the Doctor sent them to cause time distortion in 1981 and the 57th century, drawing the Timewyrm out to feed on the interference with the Web of Time. While it thought it trapped the Doctor in puterspace and planned to feed on time, the Doctor revealed that his earlier attempt to destroy her using an object he spent centuries constructing had actually altered her nature. Noting that the time stream was in flux, she returned to puterspace, no longer able to manifest in the real universe. (COMIC: The Last Word)

As Isthtar Hutchings
In 2010 Ishtar met Chris Cwej at Benny's wedding in Cheldon Bonniface, had a dalliance with him and bore 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 when his old friend, the Brigadier, was killed before his allotted 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 of the début of the Virgin New Adventures series with  Genesys. This story's placement is uncertain, but appears to occur prior to Final Genesis as the TARDIS console room is not yet the one shown in that story. However, this does not explain how the Timewyrm has returned.