C-flat,

this is what I found.

The "narrator" of the song is King Loth, ruler of Lothian. He is married to Arthur's sister Morgause, who gave him a couple of sons (one of them is Gawain). Morgause had some quarrels with Arthur, and to set things right between them, she decides to offer herself and her sons to Arthur's court (in Tintagel). She lives in Tintagel, leaving her husband Loth behind. Then, Arthur is mortally wounded in the battle of Camlan. To save him, the court decides to move him to the Otherworld island Avalon. This place is supposed to have strong healing powers, but it is only possible for certain people to enter it. Morgause knows the place, she has been to Avalon before (being Arthur's sister and all). So, she decides to accompany Arthur to Avalon. Loth rushes to Tintagel, to try to talk her out of it, because he knows that she probably won't come back if she ever goes to Avalon.

Maybe not everything I wrote here is correct, but I think it makes at least some sense.

T.