Founder of the Time Lords of Gallifrey.  The legends around his life and deeds date back to the very origins of the Time Lords, and are shrouded in mystery.  Rassilon went into a black hole, the Eye of Harmony, presumably created by the collapse of Omega's supernova, captured it, and stored it under the Panopticon, thereby providing the Time Lords with a virtually inexhaustible source of energy, capable of meeting the huge demands of time travel (4P).
 Rassilon also created the Rassilon Imprimature, a symbiotic nucleus contained within the Time Lords' genes, that was somehow connected with their ability to withstand the molecular destabilization caused by time travel, and to achieve quasi-symbiotic control of their TARDISes.  Indeed, the Rassilon Imprimature was needed to "prime" a TARDIS (6W).  Lastly, Rassilon created the three symbols of power worn by the President of the Time Lords: the Coronet of Rassilon, which linked the President's mind to the Matrix and enabled it to bend other minds to his will; the Sash of Rassilon, which protected its wearer from the deadly energies released by the Eye of Harmony; and the Rod of Rassilon (or Great Key), which controlled the access to the Eye of Harmony itself.
 Rassilon later contributed various other artefacts to the defences of Gallifrey: the Key of Rassilon, which harnessed the energy of the Eye of Harmony to power the Demat Gun (4Z) (another Key of Rassilon gave access to the Matrix (7C)); the force fields known as the Transduction Barrier, which protected Gallifrey (4Z); and the deadly Silver Nemesis, made of the living metal validium, and meant to be Gallifrey's ultimate defence (7K).  Rassilon is also credited with creating the bow ships which destroyed the Great Vampires (5P).  The book stolen by Salyavin, "The Ancient Law of Gallifrey", which contained the secret of Shada's location, was said to be an artefact created in the time of Rassilon (5M).
 Rassilon had much of the Time Lords' history preserved in documents such as the Black Scrolls (6K), and the Records of Rassilon, which used to be kept in all TARDISes (5P).
 The Harp of Rassilon was the secret key to the hidden room where the controls to the so-called Game of Rassilon were kept.  It is not known whether Rassilon was responsible for starting or ending this cruel habit of using the Time Scoop to snatch beings from various places and times and making them fight in the Death Zone for the Time Lords' amusement, but it is known that his body was later entombed in a tower located at the very centre of the Death Zone.  However, his spirit lived on, and was able to protect Gallifrey from the megalomania of Borusa by tricking him into accepting the immortality of a living statue.  (An inscription in the Tomb of Rassilon promised true immortality to anyone who would take the Ring from Rassilon's fingers.) (6K).