Rassilon was the founder of the Time Lord society on Gallifrey. There is also the Seal of Rassilon, which is used in Gallifreyan society, as well as other objects and the Tower or Tomb of Rassilon. Rassilon is credited with the technology behind the TARDIS, validium, and transduction barriers. Rassilon is known to have gone crazy, presumeably sometime nearer to the end of his natural life, possibly due to the influence of The Corruptor.
During the Time War, Rassilon was ressurected and became Lord President of the Time Lords. This incarnation of Rassilon seems to have been a maniacal and a driving force of the war machine that the Time Lords became during the Time War (The End of Time).