I've written one. (Or mostly written one. I'm still not satisfied with parts of it.) You're probably looking for existing songs though. I have a vague memory of a song by Crosby Stills & Nash about Guinnevere, but one would think there were be some trad-sounding songs as well. One reason I wanted to write a song was the fact that there don't seem to be very many songs about King Arthur or Camelot. Another reason was I really wanted to explore the question of (after Arthur's departure to Avalon) "and THEN what happened?"

2nd verse:
And his knights rode where the four winds blew,
No reason here to stay.
He dreamed a dream and they believed,
But the dream is buried in the clay.
Remembered, yes, by those who lack,
The strength that living it would take,
And Excalibur's, in the cold, cold hands,
Of the Lady of the Lake.

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