De-mat Gun
The De-mat Gun was a powerful weapon of the Time Lords of the planet Gallifrey. A remark by Borusa implies that Rassilon had at least a hand in its construction or design. It could remove the target from space-time. The gun could not be armed without the Great Key of Rassilon.
As Lord President, the Doctor used the De-mat gun against the Sontaran leader Stor and his remaining troops in the Capitol on Gallifrey. The gun apparently vaporised itself, leaving only the Great Key. The Doctor then could not remember the last few days. (DW: The Invasion of Time)
The Great Key was lost at sometime into the War, necessitating the Time Lords to research further more destructive weapons. (EDA: Alien Bodies, The Ancestor Cell)
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 the Great Key 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."
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)
