Rassilon is an ancient Time Lord, perhaps the single greatest figure in Time Lord history. Certainly he is the most important figure in the official histories of Gallifrey, and is generally considered the first Time Lord, though some believe that title more properly belongs to Omega.
Among his many achievements, Rassilon mastered time travel, ended the "Dark Times" of Gallifrey's past, battled and defeated the Great Vampires, and established many of the traditions of Time Lord society. A great many important Gallifreyan artefacts bear his name, including the Sash of Rassilon, Rod of Rassilon, Coronet of Rassilon, Harp of Rassilon, Ring of Rassilon and the Loom of Rassilon's Mouse. Despite all this, and the advanced state of Time Lord science during Rassilon's life, few certain facts are known about him. Many legends suggest Rassilon was less than a paragon of virtue, though most Time Lords accept him as their greatest hero nonetheless.
Many rumours surround Rassilon's death, but the most persistent say he discovered a form of true immortality beyond the regeneration known to Time Lords. He hid its secret with his remains in the Tomb of Rassilon at the heart of the Death Zone on Gallifrey, protected by the various creatures brought there when the Time Lords abused their power and played games with the lives of others. The Doctor discovered the truth when his mentor Borusa used him to try and discover Rassilon's secret: Rassilon did indeed discover immortality, but realised it was too dangerous a secret to share. The rumours of the Dark Tower were a trap, designed to lure those who wanted immortality to their just punishment - immortality as a living statue, imprisoned in Rassilon's tomb.