A prophecy (or in Time Travel, something that is known to have happened in the past) comes true despite all attempts to prevent it (and often because of those attempts). Often happens with an obvious (or not so obvious) Prophecy Twist on the language used in the prophecy. As old as Oedipus Rex, used by Shakespeare and Tolkien, and still fresh at least as recently as the mid-80s sitcom! Since it's Older Than Dirt, most examples rely on a Prophecy Twist. If time travel is involved, it's called You Already Changed the Past. See also Stable Time Loop and Prescience Is Predictable.