Mysterious Alternative Timelines

Sometimes sf writers make evocative use of the concept of altered timelines. Poul Anderson does this in his Time Patrol series. See here and here.

In DC Comics, the character called "Doomsday" was introduced to kill Superman - like Moriarty with Sherlock Holmes. Doomsday kills Superman in at least three continuities, on the third occasion in an altered timeline:

Superman and Doomsday fight to the death both in (i) post-Crisis comics continuity and in (ii) the Batman versus Superman film;

(iii) when, in the Smallville TV series, Rokk from the future tells Clark of an altered timeline in which Doomsday killed Clark, Clark, forewarned, is able to ensure that he is not living in that timeline.