- You may be looking for Great Key of Rassilon.
The Key of Rassilon was a Time Lord artefact which permitted entrance to the Matrix. Time Lords were arrogant in the belief that it could not be stolen or copied; they were proven wrong in the second category when the Master used a copy of the key to enter the Matrix. The Valeyard also had a copy, using it to escape through the Seventh Door to the Matrix once his true identity was revealed. (TV: The Ultimate Foe)