Note: When J.R.R. Tolkien couldn’t find a word to express precisely what he was thinking, or trying to convey in his writing, he would simply invent a word. As he was a philologist, this is not difficult to imagine. So, when conveying the supreme purpose of the fairy story, what Tolkien calls the Eucatastrophe in his essay On Fairy Stories, he could find no word that suited his definition. This led Tolkien to coin the word, eucatastrophe. It comes from the combination of two Greek words, meaning ‘eu’ for ‘good’ and ‘katastrophe’ for destruction. In other words, it is a good catastrophe, the kind of event(s) you never see coming or least expect in a story. The Gospel, Tolkien says, is the story of the greatest eucatastrophe, that joyous sudden turn from death to life. And not only is it beautiful, but it is historically true and reliable. Here is Tolkien in his own words on the great Easter Eucatastrophe.
On Fairy Stories
I would venture to say that approaching the Christian Story from this direction; it has long been my feeling (a joyous feeling) that God redeemed the corrupt making-creatures, men, in a way fitting to this aspect, as to others, of their strange nature. The Gospels contain a fairy story, or a story of a larger kind which embraces all the essence of fairy-stories. They contain many marvels—peculiarly artistic, beautiful, and moving: “mythical” in their perfect, self-contained significance; and among the marvels is the greatest and most complete conceivable eucatastrophe. But this story has entered History and the primary world; the desire and aspiration of sub-creation has been raised to the fulfillment of Creation. The Birth of Christ is the eucatastrophe of Man's history. The Resurrection is the eucatastrophe of the story of the Incarnation. This story begins and ends in joy. It has pre-eminently the “inner consistency of reality.” There is no tale ever told that men would rather find was true, and none which so many skeptical men have accepted as true on its own merits. For the Art of it has the supremely convincing tone of Primary Art, that is, of Creation. To reject it leads either to sadness or to wrath.
It is not difficult to imagine the peculiar excitement and joy that one would feel, if any specially beautiful fairy-story were found to be “primarily” true, its narrative to be history, without thereby necessarily losing the mythical or allegorical significance that it had possessed. It is not difficult, for one is not called upon to try and conceive anything of a quality unknown. The joy would have exactly the same quality, if not the same degree, as the joy which the “turn” in a fairy-story gives: such joy has the very taste of primary truth. (Otherwise its name would not be joy.) It looks forward (or backward: the direction in this regard is unimportant) to the Great Eucatastrophe. The Christian joy, the Gloria, is of the same kind; but it is preeminently (infinitely, if our capacity were not finite) high and joyous. But this story is supreme; and it is true. Art has been verified. God is the Lord, of angels, and of men—and of elves. Legend and History have met and fused. (Tolkien, The Monsters and the Critics and Other Essays, On Fairy Stories, p. 155-156).
Letters of J.R.R. Tolkien
"I coined the word 'eucatastrophe': the sudden happy turn in a story which pierces you with a joy that brings tears (which I argued it is the highest function of fairy-stories to produce). And I was there led to the view that it produces its peculiar effect because it is a sudden glimpse of Truth, your whole nature chained in material cause and effect, the chain of death, feels a sudden relief as if a major limb out of joint had suddenly snapped back. It perceives – if the story has literary 'truth' on the second plane (....) – that this is indeed how things really do work in the Great World for which our nature is made. And I concluded by saying that the Resurrection was the greatest 'eucatastrophe' possible in the greatest Fairy Story – and produces that essential emotion: Christian joy which produces tears because it is qualitatively so like sorrow, because it comes from those places where Joy and Sorrow are at one, reconciled, as selfishness and altruism are lost in Love." (Letters of J.R.R. Tolkien, Letter 89).