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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 completely remove something from time and space.
The Doctor intended to modify the gun so that it could remove millions from time and space at once, possibly sealing the Medusa Cascade if needed. (IDW: The Forgotten)
The Time Lords later refered to a time or device known as the Moment that ended the Time War. This may have been the modified version of the De-mat Gun, although precise details were unknown. (DW: The End of Time) However, the Doctor later said that it was the Great Key turning in the De-mat Gun that ended the War. (IDW: The Forgotten)