This is Tolkien's version of how King Arthur meets his end. Arthur is campaigning and about to cross into Mirkwood when he receives the news that Mordred has betrayed him. Arthur returns to face Mordred and defeat his army of mercenaries. Arthur has also recently found out that Guinevere has been unfaithful to him with one of his most trusted knights Lancelot. It is with all these recent betrayals that he faces a great battle without many of his faithful knights to help him fight.
This is an excellent poem. I wish Tolkien had finished it. He began this poem early in his career, but set it aside while he wrote The Hobbit and other projects. Most of the book is written by Tolkien's son Christopher explaining the material, and filling in the story where Tolkien did not have anything written or left clear notes about what his plans for that part of the poem were. While it is informative to read Christopher's notes on this poem, I would have preferred more of the actual piece. Unfortunately, there is nothing to be done about that.
This is a great edition to the lore of King Arthur. Read it aloud if you can find a place you are comfortable doing so, because it really makes you more involved in the story. I love the rhythm of The Fall Of Arthur. It intensifies the reading experience. It is a moving poem, and even though I am familiar with what happens to King Arthur, I still found it to be gripping.
If you like reading about King Arthur, you should not miss this poem. I highly recommend reading this book.