Dead silence fell. Out of deep valleys
fogs unfurling floated upward;
dim vapours drowned, dank and formless,
the hills under heaven, the hollow places
in a fathomless sea foundered sunken.
Trees looming forth with twisted arms,
like weeds under water where no wave moveth,
out of mist menaced man forwandered.
Cold touched the hearts of the host encamped
on Mirkwood's margin at the mountain-roots.
[J. R. R. Tolkien, The Fall of Arthur. Christopher Tolkien, ed. Houghton Mifflin (New York: 2013) p. 22. Canto I, lines 123-132.]
The Fall of Arthur is an unfinished alliterative poem that was in Tolkien's mind for at least two decades, and formative decades, at that. It was to have told, as the name suggests, the story of the death of King Arthur, following not so much Malory as the Alliterative Morte Arthure, but with Tolkien's own perspective on the poem. We get something of this right at the beginning, since the poem opens with Arthur marching to war, but not, as in the old sources, to conquer Rome, but to help defend it from the Saxons. The set-up for Camlan arises when treachery at home leads to the Saxons invading Britain itself, and Arthur must return from battling Saxons in the East in order to retake his kingdom from them.
The poem as we have it is a slim thing, only forty pages of verse and some notes and outlines, not entirely consistent or always intelligible, about how it would proceed from there. It doesn't get us very near to Camlan, ending after the first sea battle with the Saxons as Arthur returns home. That's very unfortunate, because even as it stands, the poem is excellent. The descriptions are vivid, the action scenes (particularly the aforementioned sea battle) are rousing, and Tolkien's ideas of how to treat the story are interesting in and of themselves. It's also unfortunate for extrinsic reasons, because, as Christopher Tolkien notes, his father's plans for Arthur's end, particularly as they relate to the Isle of Avalon, seem to have been closely tied in some way to Tolkien's reconceptualization of The Silmarillion and its world. (Those of good memory will remember that one of the names of Tol Eressëa was Avallónë; and in the notes for the end of Lancelot in the poem is one that cryptically ties Lancelot's end to a poem about Eärendel.)
On the other hand, Christopher Tolkien's hypothesis is that the work was just outcompeted for Tolkien's time and energy by the tumult of ideas begun with The Lost Road, which eventually gave us The Lord of the Rings; the last mention of the poem that Christopher Tolkien could find is in a letter written the same year The Return of the King was published. And so perhaps it was a fair trade, after all.