De - mat Gun The De - mat Gun , or dematerialisation gun was a powerful weapon of the Time Lord s . It could remove the target from 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 . ( TV : " The Invasion of Time " ) 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 molecule s 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 forbade 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 . ( 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 , 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 mission s were usually equipped with at least one " totem ic " 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 Dalek s , and Rassilon ' s gauntlet . They were both able to erase someone or something from history itself . ( PROSE : " Engines of War " )