The Tolkien work containing “On Fairy-Stories,” including his discussion of eucatastrophe, is Tree and Leaf.
Tree and Leaf is a collection published in 1964 that brings together Tolkien’s essay “On Fairy-Stories” and the story “Leaf by Niggle.” “On Fairy-Stories” developed from a lecture Tolkien delivered at the University of St Andrews in 1938. In it, he considers what fairy-stories are, why they matter, and how they can offer recovery, escape, and consolation.
Tolkien uses “eucatastrophe” for the sudden, joyful turn at the end of a narrative: a good catastrophe rather than a merely happy coincidence. He connects the idea to the emotional power of stories and later describes the Resurrection as its greatest example. The term is often confused with deus ex machina, but Tolkien’s concept is broader and emphasizes hope, moral meaning, and the structure of a story’s final reversal.