Berthold Schwarz

 was supposedly the inventor of the gun on Earth who was cursed by a Varkladian Knight to be unable to die without being shot by the planet's last gun.

Initially living around the 15th century, Schwarz accidentally summoned the Knight while conducting an alchemy experiment and chose to kill it, becoming cursed in the process. Over 200 years later in 1605, he attempted to finish Guy Fawkes' job of destroying the Houses of Parliament in the hopes to start a war that could wipe out every other gun and allow him to die; something that the Thirteenth Doctor and Yasmin Khan were initially powerless to stop.

Visiting Schwarz as one of the final survivors of a destroyed far future Earth, Yaz convinced the husk of Schwarz to see his 1605 self, and this convinced his younger self not to fulfil his plan, stopping the future timeline and freeing Schwarz of his curse. (PROSE: Black Powder)