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. Obsessively 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. (NA: Timewyrm: Genesys)

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 where she met Gilgamesh and, reading 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 virtually ruler of the city-state.

Ishtar used her slaves to begin placing copper plating around the walls of Kish. Once completed, this would have turned the city into a giant radio transmitter so that she could take control of every mind in the region before taking over Earth. Utnapishtim uploaded a computer virus into her, unaware that she had programmed a cobalt bomb to detonate upon her death, thus destroying the planet. The Doctor disarmed the bomb by linking her computer systems to the telepathic circuits of the Doctor's TARDIS. This allowed Ishtar to 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 was able to survive in the Vortex; thus, by doing so, he helped create the Timewyrm by inadvertently allowing the fusion of the Time Lord technology with the mind of Qataka. (NA: Timewyrm: Genesys)

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

The Timewyrm, greatly weakened, decided to travel back into the Doctor's personal timeline and strike when he was at his weakest: just after his first regeneration. During the 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 needed for 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. (NA: Timewyrm: Apocalypse)

The Timewyrm used Chad Boyle and Anthony Rupert Hemmings as 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 Bernice Summerfield and Ace, the Doctor fought the Timewyrm one more time, at which time she had downloaded herself into puterspace and plotted to corrupt the timeline. (DWM: The Last Word)

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, Jasmine Surprise Cwej-Hutchings, the girl's middle name, "Surprise", being the same as Benny's. (NA: 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 anniverary 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 managed to return, though fans can easily think of explanations for this.