The De-mat Gun was a powerful weapon of the Time Lords of the planet Gallifrey. A remark by Borusa implied that Rassilon had at least a hand in its construction or design. It could remove the target from space-time, but could not be armed without the Great Key of Rassilon.
History
As Lord President, the Fourth 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. After this, the Doctor could not remember the last few days. (DW: The Invasion of Time)
The Great Key was lost some time into the Second War in Heaven, 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 Eighth Doctor was imprisoned for over a month on an unknown planet with a Malmooth named Chantir. It was later revealed that the Eighth Doctor had planned his imprisonment to infiltrate the prison and steal the Great Key when it arrived on a transport. After escaping with it, 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) This device may have been what became known as the Moment and the Doctor used it to end the Time War. (DW: The End of Time, IDW: Don't Step on the Grass)