The Time-Gun of Rassilon was a weapon created by Rassilon shortly after the Alliance of Races' war with the Hyperion. The Gun fired time-bombs which wiped the target from history. When the gun was created Rassilon programmed it not to fire on Time Lord DNA.
The Time-Gun was kept on the Shadow Proclamation vault-world of Janizzar after Rassilon's demise/disappearance. The Cybock Imperium attacked the world and stole the Time-Gun, taking it to Earth. The Twelfth Doctor encountered them there and challenged their leader, Kronos to a game of Rassilon's roulette. The leader of the Imperium tricked the Doctor and turned the Gun on him, but when the Gun was fired it refused to shoot the Doctor due to his Time Lord DNA. The Gun backfired and instead wiped Kronos from history. (COMIC: Gangland)