Though J.R.R. Tolkien passed away in 1973, he has never really stopped publishing. For decades his son and literary executor Christopher Tolkien has painstakingly catalogued and edited his father’s papers, creating new books out of unfinished and unpublished manuscripts. Most of those tales delve deep into the history of Middle-earth, the fantasy realm where Tolkien’s best known works, The Hobbit and The Lord of the Rings series take place. Now, it’s likely that work will come to an end with one last Tolkien book. Critic Andrew Ervin at The Washington Post reports that The Fall of Gondolin, which will be released tomorrow, is likely J.R.R. and Christopher Tolkien’s swan song.

While this story may be the last Tolkien book to be published, it is actually an early tale and foundational to the author’s entire concept of Middle-earth. It was first written in 1917 while Tolkien was recuperating in a hospital from trench fever after the Battle of the Somme. “It’s a quest story with a reluctant hero who turns into a genuine hero—it’s a template for everything Tolkien wrote afterwards,” John Garth, author of a book about Tolkien’s experience in World War I tells Alison Flood at The Guardian. “It has a dark lord, our first encounter with orcs and balrogs—it’s really Tolkien limbering up for what he would be doing later.”

Christian Holub at Entertainment Weekly explains that the new book tells the tale of Tuor, a man living in an age where the world is dominated by the dark lord Melko—known in other Tolkien books as Morgoth. Only one place, the hidden Elvish city of Gondolin has resisted his reign, and Tuor is sent to find the place. He does, but so do the dark forces of Melko. In the grandest Tolkien battle scene outside of The Lord of the Rings, the author describes a mechanized army, similar to the newly introduced mechanized warfare he’d witnessed during the Great War, falling on the city.

The new book, however, isn’t just one tale. Instead, Holub explains that Tolkien rewrote the story several times, changing details and character attributes. In 1951, he took a stab at writing a more narrative version of the story versus the mythological and epic versions he produced before, but abandoned that work when his publisher showed little interest. The new volume collects all of the versions including historical notes and explanations from Christopher Tolkien.

Last year, Tolkien the Younger, who is now 93 years old, published Beren and Luthien, the second of what his father considered the three “great tales” of early Middle-earth. In the preface to that work, Christopher Tolkien suggested it was the last work he would edit, and possibly the last official work in his father’s oeuvre. So fans and literary scholars were surprised when earlier this year Tolkien announced that he was planning on publishing The Fall of Gondolin, the third and final Great Tale.

While none of the tales are quite as compelling as the journey of Bilbo or Frodo Baggins, they are remarkable for what they represent. Before Tolkien set his hobbits off on their adventures, he spent decades creating an entire world, including an entire ancient history, to couch them. It’s a feat of world-building that few, if any, other authors have achieved so successfully. “What makes The Hobbit and Lord of the Rings work as well as they do is that they are set into this cultural background with its own history and languages,” Alan Lee, who created color illustrations for the new book and the other Great Tales tells Holub. “You get so much more from those particular stories if you actually delve back and enjoy the mythology of Middle-earth. In that process of the myths changing and developing, you get all these echoes of the earlier stories running through the later ones. It makes the whole thing richer and more satisfying and more dense.”

It’s unclear whether someone else will step in and scour Tolkien’s papers for other unpublished or unfinished works, though it’s hard to imagine there’s much left to find. Since the 1970s, Christopher Tolkien has edited 24 books of Tolkien’s writing including The Silmarillion, a history of the elves, a 12-volume History of Middle-earth series, the most recent Great Tales, as well books of his father’s academic writings.

Last year, Tolkien resigned as director of the Tolkien Estate. But there’s more Middle-earth content on the way, even if it didn’t originate at J.R.R. Tolkien’s pen. Soon after Christopher Tolkien’s resignation, the estate sold TV rights to Amazon, which is in the process of creating a new television series, and possibly more than one, based in the fantasy world.