The Gauntlet of Rassilon (AUDIO: Assassins) was a weapon used by Lord President of the Time Lords, Rassilon. It contained De-mat technology that let him remove objects and individuals from history if he so wished. (PROSE: Engines of War) These same abilities allowed him to revert alterations to history. (TV: The End of Time)
History
Rassilon used the glove to disintegrate a Time Lady when she told him it might be better that the Time Lords died, showing that he also used the weapon to instil fear in other Time Lords so they didn't question him or his judgement. Along with other Time Lords, Rassilon escaped the time lock and travelled to Earth, returning them to their original form and rendering the Master unique again. He used the gauntlet to destroy the Saxon Master's facial template set upon the humans. He then also attempted to kill both the Master and the Tenth Doctor, but failed to kill either. (TV: The End of Time)
Rassilon retained a stripped down version of the glove following his regeneration and used it against the Twelfth Doctor when he returned to the preserved Gallifrey, threatening to strip him of his regenerations one by one before being overruled by the General. (TV: Hell Bent) Rassilon's gauntlet was inherited by a Time Lady who took over as Lord President following the quick departure of the Twelfth Doctor who had deposed and banished Rassilon. Rassilon reclaimed his gauntlet, along with the title of Lord President, after slaying his successor, having returned to Gallifrey leading an army of Cybermen. The timeline in which these events took place was later negated by the Twelfth Doctor. (COMIC: Supremacy of the Cybermen)