Early in The Fall of Arthur, long awaited by fans of J.R.R. Tolkien and now edited for publication by his son Christopher, an army rides to Mirkwood where they see in a storm above it, Ringwraith-like:

wan horsemen wild in windy cloudsgrey and monstrous grimly ridingshadow-helmed to war, shapes disastrous.

But this isn’t Middle-earth: it is Europe on the brink of the Dark Ages, and the army is led by Arthur and Gawain. Mirkwood is simply the old name for Germany’s eastern forests, which Tolkien borrowed for the children’s story he was writing in the same period in the early 1930s, The Hobbit.

Tolkien was a writer of endless stories. And as with most of them, The Fall of Arthur is literally endless: unfinished. It’s been lying among his vast legacy of papers, almost unknown but for a paragraph in Humphrey Carpenter’s 1976 biography and a single reference in Tolkien’s published letters. Publication follows that of the more difficult The Legend of Sigurd and Gudrún in 2009, which Christopher Tolkien probably elected to publish first because it was complete. Like Sigurd and Gudrún, The Fall of Arthur is in alliterative verse, a mode last fashionable in the 14th century. It amounts to a mere 40 pages, and was perhaps abandoned because of professional and family pressures, or in order to complete The Hobbit. But if you can abide the frustration of knowing it is a fragment, it is well worth reading. No one has done more than Tolkien to rekindle the medieval flame for the modern era; and this is his only creative contribution to the key Arthurian tradition. Compelling in pace, haunted by loss, it lives up to expectations.

Arthur is visualized as a king loyal to Rome, whose legions have left Britain. It is the era of the great westward migrations of the Germanic peoples who would eventually occupy Europe, driving the Celts to the Atlantic’s edge. At a time when Britain was itself only recently converted to “kindly Christendom,” Arthur makes crushing war on the Germanic invaders and their “olden gods.” But his is “a falling world” beset by “the tides of time”: we know he won’t win against treachery at home.

Landscape and weather, as ever, are Tolkien’s allies in raising or releasing tension. Storm in sky and sea matches human passions and conflict; waves grind boulders “with ogre anger.” The narrative flits from big military scenes to personal close-ups, then surges rapidly back again. There is of course no hobbit levity. The main players are stark personifications of principle, or its lack.

Gawain is less a personality than a piece of granite, a man “whose glory waxed / as times darkened.” Lancelot has an impulsive, glad optimism out of tune with the times. His love of Guinevere is misplaced: she is a “greedy hearted” hoarder of gold or love, whose feelings ill-suit the Round Table’s ethos of public service, honour and chivalry: a “lady ruthless, / fair as fay-woman and fell-minded / in the world walking for the woe of men.” Mordred is a slave to his lust for the Queen, finding no outlet for his thwarted energies except in scheming action.

By the time he began The Fall of Arthur, Tolkien was already master of the alliterative mode, in which the two or three most important words in each line begin with the same sound, and in which each line contains a pause midway—a “caesura” represented by extra spacing. No one should be surprised to encounter inversions of word order and archaism: targes, blazons, vawards, dromonds and broidery all in the space of a dozen lines. In his generous and illuminating commentary, Christopher Tolkien quotes a 1934 letter from a friend of his father’s who, reading Arthur on a train, “took advantage of an empty compartment, to declaim him as he deserves.” Out loud is certainly best; a train is strictly optional.

Readers of The Lord of the Rings will be familiar with the style. “We heard of the horns in the hills ringing”—that was how the poets of Rohan evoked the clarion call to action. In The Fall of Arthur almost the same line appears, with a downbeat difference: Arthur’s army in Mirkwood “heard a horn in the hills trembling.” It’s the horn of a messenger hotfooting it from Britain to tell the king he’s been betrayed by Mordred, left behind as his steward.

Arthur’s first thought is to summon Lancelot from his home in France. But—as we learn in flashback—Lancelot has been banished following his adultery with Guinevere, and Gawain argues mistakenly that he can’t be trusted. A fog of misunderstanding between Mordred’s enemies leaves them fatally vulnerable.

The poem breaks off at the most frustrating of junctures, just as Arthur and Gawain contemplate a Gallipoli-style attack via Britain’s white cliffs. Arguing against the unequal assault, Arthur is the model of the wise and just military leader, like Aragorn in The Lord of the Rings but very unlike the generals Tolkien’s generation had known in World War I, who wasted lives in the tens of thousands in ill-conceived and futile assaults. But the attack goes ahead, with dire consequences.

The Fall of Arthur deals almost exclusively with personal and military manoeuvering. Yet this is a world on the edge of fantasy, in which Gawain carries a rune-enchanted sword and talks of summoning aid from the “fay” people of the Isle of Avalon. And it is in Avalon that, in a curious and tantalizing fashion, The Fall of Arthur dovetails with Tolkien’s mythology of Middle-earth.

The mysterious Avalon to which Arthur is taken to die is often regarded by Arthurian chroniclers to be Glastonbury, a very real place in the West of England. But Tolkien has none of that. As attentive readers know, Middle-earth was never meant to be another world, but our own in an earlier era. In 1916, long before he envisaged The Hobbit or The Lord of the Rings, Tolkien had begun writing the “Lost Tales” (prototypes of The Silmarillion), which he imagined being told to a mortal mariner who reached the Lonely Isle of the Elves in the uttermost west. In The Fall of Arthur, when Tolkien writes of Avalon, he means that same elvish island.

Most tantalizingly of all, notes for the unwritten ending have Lancelot sailing to Avalon in Arthur’s wake. This is emphatically not what happens to Lancelot in the medieval sources: as Christopher Tolkien explains, Sir Thomas Malory’s 15th-century Le Morte d’Arthur has him living in austerity in Britain, to pine away finally after the death of Guinevere. Was Tolkien’s decision to send Lancelot to Avalon, reconfigured as the island of the High Elves, simply self-indulgence by an author whose writings were so often drawn into the orbit of Middle-earth—something no more significant than when a whale carries the dog-hero of his children’s story Roverandom within sight of the Bay of Faërie and away again? Or was Tolkien actually planning to use The Fall of Arthur to introduce readers to his Silmarillion stories, making Lancelot the mariner who would learn those ‘lost tales’ from the Elves of Avalon? He had been working on those stories for nearly two decades, and certainly in the 1930s he was casting about for ways to get them published. But we will never know if he did indeed see Arthur this way. Very soon after the poem was abandoned, The Hobbit found its way to the publishers, and that far more populist story proved to be the doorway through which Tolkien could begin to smuggle his older, epic vision of elvendom.