De-mat Gun

The dematerialisation gun, more often simply called the De-mat Gun or D-Mat, was a powerful Time Lord weapon that could remove its target from time.

Effects
When its trigger was pulled, the De-mat Gun would fire a beam that erased its target from reality itself. (TV: The Invasion of Time) It did so by scanning its target's timeline, identifying every particle that ever interacted with their body, and then using its superpowerful quantum computer, linked to the Homeworld's causal nexus, to 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, 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 forbade its use in battle under normal circumstances. The De-mat gun was only acceptable to use as a final resort, and even then, it required the Great Key of Rassilon to operate, preventing it from being used if it were stolen. (TV: The Invasion of Time)

History
As Lord President, the Fourth Doctor hypnotised Rodan, assisted by K9 Mark I, into building a De-mat gun. He 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 in Heaven, leading the Time Lords to research further more destructive weapons. (PROSE: Alien Bodies, The Ancestor Cell, The Book of the War) Nevertheless, the D-Mat became an important symbol among the House Military during the War: "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)

During the Last Great Time War, Rassilon's gauntlet and the Daleks' Temporal Cannon were De-mat weapons with similar effects to the De-mat Gun. (PROSE: Engines of War) During the Time War, the Eighth Doctor made a plan to use the Great Key of Rassilon to modify the De-mat Gun so it could remove millions from time and space at once, possibly sealing the Medusa Cascade if needed. To steal the Great Key, he caused himself to be imprisoned for over a month on a planet with Chantir. The Doctor would successfully create and use the modified De-mat Gun, (COMIC: The Forgotten) the Moment, to end the Time War, (TV: The End of Time, COMIC: Don't Step on the Grass) but as a side effect it erased his memories of Chantir, among many other things. (COMIC: The Forgotten) By another account, the War Doctor stole the weapon from the Omega Arsenal rather than build it. (TV: The Day of the Doctor, PROSE: Engines of War)