Great Key of Rassilon

The Great Key of Rassilon was one of the Artefacts of Rassilon. It was rarely in the possession of the Lord President. The Chancellor typically held the Great Key. A forbidden weapon known as the De-mat Gun was activated by the Great Key. (TV: The Invasion of Time)

History
Toward the end of the Last Great Time War, the Eighth Doctor was imprisoned for over a month on an unknown planet with a Malmooth named Chantir; it was later revealed that the Doctor had planned his imprisonment to infiltrate the prison and steal the Great Key when it arrived on a transport. After escaping with the Key, the Doctor explained it had once been part of a weapon called the de-mat gun, a weapon that could "...remove something completely from time and space."

When asked if it would "...become another de-mat gun... a weapon of destruction...?" the Doctor replied, "Yes. But not as small as a gun. The de-mat gun removes people from time and space - one at a time. I may need to create something that removes millions from time and space - at once. This key will lock the Medusa Cascade forever — if we need to." (COMIC: The Forgotten)

According to one account, the Doctor created the Moment with the Great Key. This wiped out millions from time and space at once and ended the Last Great Time War. (COMIC: Don't Step on the Grass) Another account claimed that the Moment was created by the Ancients of Gallifrey and placed in the Omega Arsenal. This account showed the War Doctor expressing unfamiliarity with the Moment's controls and sentient operating system, and that he ultimately didn't use it on Gallifrey; he only gave the appearance of the Time Lords and Daleks wiping each other out through placing Gallifrey in a pocket universe, which caused the Daleks to accidentally kill one another in the crossfire when Gallifrey disappeared. (TV: The Day of the Doctor)

Grande Chave de Rassilon