Exist verb – To have life. Occur is a synonym for exist in prevail topic. In some cases you can use "Occur" instead a verb "Exist", when it comes to topics like be living. popular alternative Nearby Words: existence, existing
Butterfly of Doom: In "The City on the Edge of Forever", Edith Keeler's death must occur or else it will cause an alternate timeline where Germany wins World War II and Starfleet does not exist.
All of the deaths in the show are of this variety and it is explained early on that the gravelings exist solely to make sure the circumstances leading to the deaths occur.