Cyber-President was a position attained by Rassilon after he allied with the Cybermen, led by the Cyber-Controller, at the end of the universe following his banishment from Gallifrey. Working alongside a Cyber-Leader, Rassilon returned to Gallifrey and conquered the planet with the aid of his new allies. Having the current Lady President killed, Rassilon reclaimed his gauntlet and the title of President. He welcomed the Twelfth Doctor when he returned to Gallifrey.
As the Doctor tried to gather his remaining Gallifreyan allies, he learned that Rassilon intended to use the accumulated energy of the captured Time Lords to literally regenerate the universe to fit his vision, only for his plan to be subverted by the Cybermen. The Cybermen placed the Doctor and Rassilon into the Cyberiad. Inside, Rassilon realised his mistake and formulated a plan with the Doctor to use the regenerative energy of the Time Lords to undo what the Cybermen did to the universe. Rassilon helped the Doctor to realise his plans, and the universe was regenerated, setting things to how they were before Rassilon had met the Cybermen.
Rassilon was once again left in his bowship at the end of the universe. The Twelfth Doctor wondered if Rassilon was able to remember the events with the Cybermen like he could. (COMIC: Supremacy of the Cybermen)