DOCTOR WHO IN DETAIL

The official records credit Rassilomn as the founder of modern Time Lord civilisation. During his time Rassilon was an engineer and architect who, with protection from what has become know as the Sash of Rassilon, was able to harness power of a black hole, the Eye of Harmony. The sash balances all things inside a black hole that they may neither flux, nor wither, nor change their state in any measure. Using an ebonite rod, the Key of Rassilon, Rassilon was able to to seal it within Gallifrey. The power from the Eye of Harmony powers all TARDISes, and gave the early Gallifreyans the power for their time travel experiments.

Official records also tell of how Rassilon saw how the early Time Lords misued their powers in the Death Zone and sealed the zone by setting up a device that would jam teleports and forbade the use of the time scoop. There were myths and legends to the contrary that said Time Lords rebelled at Rassilon's cruelty and locked him away inside the Dark Tower inside the Death Zone in eternal sleep.

Rassilon possessed perpetual regenerations making him immortal.  During the Time War Rassilon was awoken from his sleep and was reinstated as the President of Gallifrey to lead the fight against the Daleks.