*Spoiler* If you haven't gotten that far in Doctor Who. But when I found out that he was The Face of Boe, I kind of figured it out before, but it was still a big surprise. The people of BBC really do know what they are doing.
Others have commented the assassin is a time traveller. Time travel has nothing to do with it! Affect an unrelated thing in such a way that the knock on effect leads to the death of the intended victim.
My fiance's phone has the lightsaber sound assigned for my incoming texts. But his female bff (and groomsgirl) gets the TARDIS sound. Trying to not be jealous.) -- lightsaber sound for him, tardis sound for her - how have I not thought of this?