Proposed by Lord President Rassilon as the last resort to winning the Last Great Time War, the Ultimate Sanction involved the creation of a paradox so severe that the resulting spatial-temporal rupture would rip the Time Vortex apart.
As this would be a suicidal measure, Rassilon devised a way of allowing the Time Lords to escape the disintegration of Creation by having the whole race shed their corporeal bodies and become creatures of consciousness alone, ones that would escape the effects of time and of cause and effect. (TV: The End of Time)
This plan was thwarted when "all thirteen" of the Doctor's incarnations froze Gallifrey and placed it inside a pocket universe, causing the Daleks who were firing on Gallifrey to accidentally kill one another in the crossfire when it vanished; the War Doctor, who was fighting in the Time War, lost his memories of these events due to being out of sync with his time stream and from the outside, it appeared as if the Daleks and Time Lords had destroyed each other. (TV: The Day of the Doctor)
Nevertheless, Rassilon nearly succeeded when his sleeper agent, the Master, used a Whitepoint Star diamond to send a physical rhythmic signal of four beats into time and space. Rassilon locked onto that signal. He used it as a homing beacon to break Gallifrey and every participant of the Last Great Time War out of the time lock and begin the End of Time.
Rassilon's plan was again foiled when the Tenth Doctor used Wilfred Mott's old service revolver to destroy the diamond. Rassilon tried to kill the Doctor for this, but was attacked by the Master, saving the Doctor. The Master assaulted the Time Lords as Gallifrey was plunged back into its last day. (TV: The End of Time)
This event was foreseen by the Ood (TV: Planet of the Ood), Carmen (TV: Planet of the Dead), the Trickster (TV: The Wedding of Sarah Jane Smith) and apparently even Lord Azlok of the Viperox. (TV: Dreamland) They all tried to warn the Doctor through prophecies, which also informed him of his upcoming regeneration.
News of the Ultimate Sanction, or at least that the High Council was planning something, was brought to the General by Androgar, but the General dismissed the Council's plans as having "already failed". (TV: The Day of the Doctor)
Behind the scenes
- Rassilon planned for the Time Lords to escape the effects of the Ultimate Sanction by becoming "creatures of pure consciousness". This is exactly what the Celestial Intervention Agency did in the aborted timeline of the War, becoming the Celestis. (PROSE: Alien Bodies)
- In The End of Time, it is implied that the Doctor used the Moment when he became aware of the Ultimate Sanction and that most of the Time Lords had agreed to it, suggesting that the Time Lords' moral corruption was one reason the Doctor resorted to such drastic measures. In The Day of the Doctor, the Ultimate Sanction is only given a passing reference. Both episodes, however, show that the Doctor acquired the Moment prior to Rassilon gaining the approval of the other Time Lords.