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 (and invented the TARDIS), 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, though most Time Lords accept him as the great hero the "official histories" suggest.
Many legends suggest Rassilon was less than a paragon of virtue, especially in his dealings with Omega. While nearly all accounts agree the pair worked together (in some stories with a third figure known as "the Other") to give the Time Lords mastery over time, they differ on whether he manipulated Omega into performing the risky stellar detonation that led to him being trapped in the universe of anti-matter. He is generally created with chaining the resulting black hole beneath the Panopticon, creating the Eye of Harmony, though again, accounts differ as to whether he did so via his own devices, or if he simply followed Omega's instructions.
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.