The De-mat Gun, or dematerialisation gun was a powerful weapon of the Time Lords. It could remove the target from space-time, but required the Great Key of Rassilon to arm it.
Effects
When its trigger was pulled, the De-mat gun would fire a beam that erased whatever it hit from reality itself. [source needed] Despite its small size, it was a superpowerful quantum computer linked to the Homeworld's causal nexus. It would scan its target and read their complete timeline, identifying every particle that ever interacted with the target's body. It would then remodel everything else in creation without the target. From the user's perspective, the machine didn't harm anything, but from the victim's point of view, they were suddenly built out of molecules that the universe didn't remember. Despite having no physical form, no past, and no present in which to act, the victim would nonetheless continue to exist as a "naked soul". (PROSE: The Book of the War)
The gun's power frightened the Time Lords, and they forbid its use in battle under normal circumstances. The De-mat gun was only acceptable to use as a final resort, and even then, required the Great Key of Rassilon to operate, preventing it from being used if it were stolen. [source needed]
History
A remark by Borusa implied that Rassilon had at least a hand in its construction or design.
As Lord President, the Fourth Doctor used the De-mat gun against the Sontaran leader Stor and his remaining troops in the Capitol during their invasion of Gallifrey. The gun apparently vaporised itself, leaving only the Great Key. After this, the Doctor could not remember the last few days. (TV: The Invasion of Time)
The Great Key was lost during the opening strikes of the War, necessitating the Time Lords to research further more destructive weapons. (PROSE: Alien Bodies, The Ancestor Cell) Nevertheless, "D-Mat 'em 'til they aren't, and hit 'em when they weren't" was a rallying call on at least one House Military front, and troops in forced regen missions were usually equipped with at least one "totemic" D-Mat weapon; faux D-Mat technology was a burlesque device used by eccentric Fourth Wave units. (PROSE: The Book of the War) These de-mat guns were stored on Gallifrey Nine. (PROSE: The Taking of Planet 5)
Toward the end of the Last Great Time War, the Eighth Doctor was imprisoned for over a month on a planet with a Malmooth named Chantir. It turned out the 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 noted the Great Key had been part of a weapon called the De-mat Gun, which 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. (COMIC: The Forgotten) This device may have been what became known as the Moment, and the Doctor planned to use it to end the Time War. (TV: The End of Time, COMIC: Don't Step on the Grass) However, according to one account, the War Doctor did not understand the workings of the Moment shortly before activating it, and appeared to steal the weapon from the Omega Arsenal rather than build it. (TV: The Day of the Doctor) Prior to the Tear of Isha being deployed, a Time Lord cardinal in the Capitol council chamber along with the Castellan also identified the Moment as a weapon that sat in the Arsenal along with the Tear. (PROSE: Engines of War) The Tenth Doctor said that he did fire the De-mat gun he made at some point in the future, but its side-effect of memory erasure caused him to forget nearly all of his encounter with Chantir, among many other things. (COMIC: The Forgotten)
During the War, two other de-mat weapons were used: the Temporal Cannon, by the Daleks, and Rassilon's gauntlet. They were both able to erase someone or something from history itself. (PROSE: Engines of War)