The De-mat Gun was a powerful weapon of the Time Lords, it could remove the target from space-time. It could not be armed without the Great Key of Rassilon. (DW: The Invasion of Time)
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Doctor with the D-Mat gun. (DW: The Invasion of Time)
It was lost at sometime into The War, necessitating the Time Lords to research further more destructive weapons. (EDA: The Ancestor Cell)