Even in its fragmentary and unfinished form — about 40 pages of text, a bit more than four cantos of what was evidently intended to be a much longer narrative poem — “The Fall of Arthur” is recognizably the work of J. R. R. Tolkien, veteran of the Great War and future author of “The Lord of the Rings.” This is an incomplete but highly compelling retelling of perhaps the most famous and familiar legend in the British tradition, a retelling the author’s son and literary executor, Christopher Tolkien, believes was begun in the early 1930s and abandoned by 1937. Perhaps not coincidentally, that was the year “The Hobbit” was published, offering readers their first glimpse of a fantasy universe that would shape the imaginative life of the 20th century and beyond.

With Christopher Tolkien now in his late 80s, this could be nearly the final work to emerge from his father’s posthumous archives. One feels a note of personal regret and disappointment in Christopher’s copious notes to “The Fall of Arthur” that is not always present in his editorship of many other incomplete Tolkien works. Christopher is always disapproving about his father’s famously illegible handwriting, but he goes much further here, describing this poem as “one of the most grievous of his many abandonments.” Read it and you’ll see why.

In reinterpreting and synthesizing a wide range of medieval and modern sources, “The Fall of Arthur” begins to reimagine the Arthurian world in startling fashion, prefiguring many of the themes and images that recur throughout Tolkien’s later imaginative work, as well as its language. It also contains a few intriguing hints that Tolkien saw his own created universe, the “Lord of the Rings” legendarium, as explicitly connected to the mythology of other places and times; as Christopher puts it, “with the stories and the dreams of peoples who dwelt by the coasts of the great Western Sea.”

This long poem is also experimental in a different way — one that may pose significant obstacles for modern readers and creates, I think, an internal dissonance or instability within the work that may help explain why Tolkien never finished it. The poem is about a mythological or pseudo-historical Celtic British king who would have lived around the sixth century A.D. But it is written in modern English and in the “alliterative verse” style of the later Middle Ages — with two linked half-lines, separated by a caesura — familiar to readers of “Beowulf” or “Sir Gawain and the Green Knight.” (Tolkien buffs will already know that alliterative verse recurs many times in his work, including the poetry of the Rohirrim, the Anglo-Saxon-like horse people of “Lord of the Rings.”)

Occasionally the deliberate archaism and backward grammar of “The Fall of Arthur” gets in the way of intimacy and understanding and casts a scrim of phony antiquity over the whole enterprise. Here is Arthur, wondering aloud to the loyal Gawain whether they should summon the disgraced Lancelot to help fight Mordred, Arthur’s treacherous nephew. (In this version as in some others, Lancelot has been banished to France after his adulterous affair with Guinevere, Arthur’s queen.)

. . . Best meseemethswift word to send, service cravingto their lord of old. To this leagued treasonwe must power oppose, proud returningwith matchless might Mordred to humble.

But such passages are more the exception than the rule. This is an action-packed, doom-haunted saga, full of vivid natural description, plunging us right into the middle of an apocalyptic war with almost no exposition. Arthur’s British kingdom, a Western and distinctly Christian realm of goodness and light, is endangered by a great shadow out of the East, which is at first simply presented as the land of the Saxon raiders but then comes to seem a vale of darkness whose warriors are spectral and perhaps supernatural:

The endless East in anger woke,and black thunder born in dungeonsunder mountains of menace moved above them.Halting doubtful there on high saw theywan horsemen wild in windy cloudsgray and monstrous grimly ridingshadow-helmed to war, shapes disastrous.

That sounds less like a foe to be found among the Germanic tribes of early medieval Europe than like the fell hosts of Sauron, riding forth from Mordor with the dreaded Nazgûl leading the way on their winged steeds. Arthur wins great victories, but his campaign of eastern conquest, and indeed the whole poem, is shot through with a characteristic Tolkienian sadness, that sense of a golden age of magic and wonder that is sliding into the sunset. This is found throughout his works — “The Lord of the Rings” is specifically such a story — and may be derived, as many have supposed, from Tolkien’s experiences seeing so many of his generation die in the trenches of Belgium. When Arthur gets word from home that Mordred has betrayed him, seeking to seize both his kingdom and his queen, he sits silently and reflects:

Now from hope’s summit headlong fallinghis heart foreboded that his house was doomed,the ancient world to its end falling,and the tides of time turned against him.

Arthur turns his ships toward home, and a crackerjack sea battle at the cliffs of Dover that features the most dazzling alliterative verse of Tolkien’s career. (“Oars were splintered. Iron clave timber, / and ropes were riven. With rending crash / masts dismantled as mountain-trees / rushed down rattling in the roar of battle.”) As for Guinevere, the Helen of this particular war, if she is in some respects a stock female character — wily and manipulative, more than willing to trade on her sexuality for power — she is nonetheless an unusually robust woman in Tolkien’s universe, full of vigor and intelligence.

Christopher Tolkien contributes an extended scholarly essay on how his father’s poem — drawn largely from an obscure Middle English poem known as “the alliterative ‘Morte Arthure’ ” (to differentiate it from other similarly titled works) — relates to the existing Arthurian tradition. But for Tolkien buffs, the big draw will undoubtedly be Christopher’s exploration of the relationship between “The Fall of Arthur” and “The Silmarillion,” Tolkien’s great compendium of legendary material. In particular, Christopher suggests that Tolkien viewed the isle of Avalon, to which Arthur departs after being mortally wounded by Mordred, as being the same place as the Elvish “Lonely Isle” of Tol Eressëa that lies at the outer limits of the human world in his own work.

Christopher Tolkien suggests that his father abandoned “The Fall of Arthur” partly because of the demands of work and family and partly because of “the great sea changes that were taking place” in his conceptions as he completed “The Hobbit” and worked more intently on the mythic material that would give rise to “Lord of the Rings.” I see a more specific possibility. If there was ever any historical cognate to Arthur, he was a Celtic Briton who spoke a language ancestral to modern Welsh and Cornish. To write about him in the Germanic or Anglo-Saxon verse style of later centuries — that is, in a style brought to Britain by precisely the people Arthur sought to destroy — can only have struck this eminent philologist as an uncomfortable linguistic and historical pastiche.

As Tolkien made clear on many occasions, he wanted to create a national mythology for England (as opposed to the quite different notion of Britain), and to return a sense of magic and mystery to that nation’s overly domesticated countryside. He would have been acutely aware that Arthur was at best a borrowed figure in the English tradition, a romanticized remnant of a conquered Celtic culture. Christopher Tolkien writes that “The Fall of Arthur” emerged from a “time of great creative upheaval” in his father’s life. As I see it, Tolkien wrote this absorbing and unfinished poem to test out the idea of connecting his own imaginative universe to Arthurian legend before deciding, consciously or not, that his work was better off standing on its own.