Zardoz (1974) ending / spoiler
It's discovered that the book is The Wizard of Oz. This revelation leads to the death of the Immortals (and most of the other major characters) at the hands of the savages from the outlands. Arthur reveals he was Zardoz all along, and he dies too.Zed and Consuella escape, and (shown via "flash forward,") live in a cave together, have a son (who leaves), grow old, die, and turn into skeletons.
Submitted by Malebolgia