During the time war Rassilon establishes a number of different time lines. Each time line exists in its own pocket universe so that they cannot influence each other.
Rassilon then plays out a number of different scenarios in each universe to see which results in the time lords winning the war. Each scenario ends with the same outcome, the destruction of Gallifrey, all except one. The successful scenario is the one where Ruth is the first doctor. Her mission was to stop the war ever happening by using coercion, assassination or any other means.
Rassilon maintains control of the pocket universes using the time less child. The timeless child has been exposed to the untempered schism in the same way the master was as a child. To stop the timeless child being driven insane, Rassilon keeps them in a perpetual state of childhood.
Rassilon intends to use a white point star to transfer himself to Ruths timeline but as the events of “The sound of drums” play out Rassilon uses the star to instead transfer Gallifrey out of the time war. When this attempt is foiled Rassilion finds himself and Gallifrey frozen in its own pocket universe (see The day of the Doctor).
From this pocket universe Rassilon finds he can no longer control the alternative time lines, the timeless child has been killed during the Dalek attack. When the Doctor banishes Rassilon during Hell Bent he establishes contact with the last of the Cybermen and agrees to become a converted cyber leader (see the comic, Supremacy of the Cybermen). It is the converted Cyber Rassilon that Captain Jack is referring to.
With no one controlling them the alternative time lines begin to bleed into one another, as a result the timelines become intertwined, allowing for the same person to exist in each other’s time line. Rassilon sees an opportunity to cyber upgrade all the timelines in his own image and surrenders to the Doctors pleading with them to hand him over to the Jadoon to stand trial for war crimes.
What he really wants is to take control of their Tardis’s and use the eye of harmony and his own regenerative energy to cyber upgrade all of time and reality.
Both doctors suspect what Cyber Rassilon is doing and a fight breaks out between Ruth and Cyber Rassilon. Both fall into the eye of harmony. Cyber Rassilon, now in the eye, tries to carry out his plan but Ruth uses her regenerative energy to cancel out Cyber Rasilions. Ultimately both perish and the alternative time lines cease to exist.
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