Rassilon is a fictional male Timelord timelord in the Doctor Who series. The character was portrayed on screen by Timothy Dalton. His allegiance / loyalty is to The Timelords. Lord Rassilon is the supreme leader of the Timelords on the planet of Gallifrey. Like the Doctor, he has the power of regeneration. When Gallifrey was hidden from time and space because of an attack by the Daleks, he stayed on Gallifrey. When Gallifrey was bought back into the here and now, Rassilon came too. He doesn't often leave his planet.
He was played by Timothy Dalton in the two-part episode that saw the end of David Tennants time as the Doctor. Timothy Dalton is more famous for playing the suave British secret agent, James Bond, 007.