Arthur must fight a huge knight or come back later and say what women most desire. An ugly woman will give the answer if Arthur marries her to one of his knights. Gawain agrees, leaves it up to her to be beautiful by day or night, and lifts the spell
This story is also found in the fifteenth century romance "The Wedding of Sir Gawen and Dame Ragnell," viewed by Sir Frederic Madden to be the source of the ballad.
The theme of the "loathly woman" is, of course, common, but certain scholars have tried to link every ballad on this theme (e.g. "The Half-Hitch") to this ballad. The links are usually very dubious. We should note that, apart from the dubious piece in Niles, the only extant version of this ballad is the copy in the Percy folio.
On the other hand, the connection between this ballad and "Dame Ragnall" are hard to deny. If this is not a recomposition of that romance, it certainly derives from the same immediate source.
The romance is found in only one manuscript, Bodleian MS Rawlinson C 86, which has lost a leaf containing probably about 70 lines after line 628.
The manuscript is generally regarded as dating from about 1500. The poem itself is probably 50-150 years older -- though the very confused writing makes things harder. I observe that, in the first 60 lines, there the name "Arthur" is spelled "Arthoure," "Arture," "Arthoure" again, "Arthure, and "Arthour."
Several other ballads also derive loosely or from Middle English romance, or from the legends that underly it, examples being:
* "Hind Horn" [Child 17], from "King Horn" (3 MSS., including Cambridge Gg.4.27.2, which also contains "Floris and Blancheflour")
* "King Orfeo" [Child 19], from "Sir Orfeo" (3 MSS., including the Auchinlek MS, which also contains "Floris and Blancheflour")
* "Blancheflour and Jellyflorice" [Child 300], from "Floris and Blancheflour" (4 MSS, including Cambridge Gg.4.27.2, which also contains "King Horn," and the Auchinlek MS, which also contains "Sir Orfeo") - RBW