"I'm The Doctor, bitch." You say as you lean the chair back, throw your feet up on the table, and your hands behind your head. "No you're not, you're not even a companion. We've contacted him." You nearly fall to the floor out of shock.
The butterfly effect. (PROMPTUARIUM » writing prompt)
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.
One day, you wake up as a human lie detector who is able to accurately tell when someone is lying. Your ability tells you that everything your best friend says, including their name, is a lie. (Pinning for the description not the actual prompt)