Mercy guns were weapons of Time Lord design. These guns were capable of remembering who they shot — regardless of the time that had elapsed since the target was hit by the original shot, unaffected even if they regenerated since then — with the first attack stunning the target and the second killing them, the reasoning being that you deserved to die if you had to be shot by them a second time. They were small and golden shaped machines that were very difficult to destroy. Lord Roche had a mercy gun that was once accidentally fired at him, but the Third Doctor claimed that he wouldn't even countenance the name for that device, let alone use it. (PROSE: The Suns of Caresh)