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. After Roche regenerated into a double of the Third Doctor, Troy Game used the mercy gun to test whether the Doctor was the Doctor or had been replaced by Roche because the Doctor was continuing with Roche's plan to save Caresh despite Troy having just discovered that this plan would destroy half of Caresh with the solar radiation, not taking into account that the Doctor hadn't known about the radiation issue. (PROSE: The Suns of Caresh)