Pandora
Pandora was an ancient female Time Lord that existed early in Gallifreyan history and was the first female President of her people.
According to legend, she sought to overthrow the traditions of Rassilon, to lead Gallifrey to war and attempted to use the power of time travel to reshape the Web of Time to suit her own design. She was also known to have taken an offworld bodyguard who killed all those that opposed Pandora's will. To this end she declared herself Imperiatrix. In time, however, the High Council managed to convince the bodyguard to betray her master on the condition that the Time Lords would help improve her people's livelihood. However, once Pandora was betrayed, the High Council took the bodyguard back to her planet where she was placed in an eternal 6.8 second time loop from which there was no escape.
The captured Pandora was dispersed and executed in such a thorough manner in that she never existed in the first place. This was why she existed only in legends and not in history. The dispersal chamber itself was located in the old Capitol where it was destroyed in Gallifrey's civil war. The new Capitol was built over the ruins which became the Vaults. The legends said that Pandora's dying screams still could be heard in the Vaults where she eternally cries out for vengeance. (BFA: Lies)
From her last remaining presence in The Matrix, Pandora manipulated Time Lord genetics through the centuries until the end result was Romana. During her time at the Time Lord Academy, Romana visited the Vaults, enticed by the voice of Pandora. She told Romana of her Imperiatrix Imprimatur, and of her destiny to rule Gallifrey under her influence. Disturbed by the encounter, Romana told her tutor, Irving Braxiatel. He used his skills to hypnotise her into forgetting both Pandora and himself. Some time later, during her travels with The Doctor, the torture by an agent of the Black Guardian unleashed the hidden memories, forcing Romana to regenerate voluntarily, in order to repress them again. While Romana continued to travel with the Doctor for a time before exiling herself in E-Space, Pandora apparently engineered her return to Gallifrey and her ascension to presidency after the disasterous President Flavia was deposed. Braxiatel had supported her campaign in order to closely observe her for any sign of continued influence by Pandora (though Romana was unawre she had ever known Brax before).
During her presidency, and accompanied by her own offworld bodyguard, Leela, Romana again encountered Pandora - using the energy of Coordinator Narvin to take the physical form of Romana's first incarnation. Through this meeting Romana learned of her own history. Pandora was then trapped in the Matrix by K-9, but only temporarily. (BFA: Lies) Shortly afterwards, Castellan Wynter came into contact with Pandora and was driven mad. As a more stable solution, Brax absorbed the past and present aspects of Pandora from the Matrix before going into exile to isolate her from other Time Lords (especifically the most ambitious of them - Darkel), leaving the future aspect still present. In spite of the obvious risks involved, Romana continued to seek advice from Pandora, through K-9, on the various challanges facing her rule. Pandora's advice was generally to declare herself Imperiatrix and to rule by decree. Darkel meanwhile, had been recieving guidence from a disembodied voice in the Vaults, and as a result challanged Romana for the presidency. Before an election could be held, Romana declared herself Imperiatrix and dismissed the High Council. Pandora than materialised (again in the form of the first Romana) claiming superior rights to the title. (BFA: Imperiatrix)
A short but bloody civil war ensued ranging the more reactionary forces of Pandora (including Darkel, Valyes, Janartis and - superficially - K-9) against the more progressive elements and those concerned with maintaining the technicalities of the succession (Romana, Leela, Narvin, Hallan, Annos and - waveringly - Matthias). The war was won not by military might, but rather by Romana and K-9 tricking Pandora back into a Matrix partition, and then destroying the Matrix entirely, killing this aspect of Pandora. (BFA: Warfare) The remaining aspects of Pandora became a force of pure hate, and when Brax returned to Gallifrey and was cajoled into releasing the force by Darkel, it consumed Darkel rather than Brax. With still a tiny part of Pandora remaining in his mind, Brax then returns to his exile, the threat apparently neutralised. (BFA: Mindbomb)