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)

The First Doctor once had a similar key to the Matrix. It was collected by the Graak on a mission from the Master to free one of the Doctors' incarnations. (GAME: Destiny of the Doctors)

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