- The Great Key is sometimes used to refer to the Rod of Rassilon.
The Great Key (also known as the Great Key of Rassilon) was one of the Artefacts of Rassilon, which was never (except in rare circumstances) in 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. (DW: The Invasion of Time)
Toward the end of the Last Great Time War, the 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 that it was once 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." (IDW: The Forgotten)
It is unclear whether the Doctor created a superior De-mat Gun which could wipe millions from time and space at once (by locking the Medusa Cascade or not) or if the Great Key alone could lock the Medusa Cascade (possibly wiping millions from time and space). (IDW: The Forgotten) However, the Time Lords hoping to escape the Last Great Time War mention the Doctor possessed an item or device known as The Moment, which was used to seal the Time Lords, the Daleks, and many other menaces on the Hell that the War had become. It is possible the Moment was a byproduct of the technology used in the De-Mat Gun, and therefore needed the Great Key to be used.