Timewyrm

Qataka was originally a powerful being who escaped the planet Anu, to Earth where she fronted herself as the goddess of sex, Ishtar. When the Seventh 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.

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 won. In doing so, the Timewyrm became trapped inside Adolf Hitler's mind.

The Timewyrm was then pursued to the planet Kirith in the far, far future before being hunted down within 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). The baby was taken by the Seventh Doctor to Cheldon Bonniface on the planet Earth. He gave the baby to Emily and Peter Hutchings and asked that they name her Ishtar.