After Arthur had sailed for France, Mordred waited a reasonable interval, then circulated a forged letter saying that Arthur had died at Launcelot’s hands, and he made himself king at Canterbury. He even tried to marry Guenevere, but she locked herself in the tower of London and wouldn’t come out, and the Bishop of Canterbury fled to Glastonbury because Mordred had threatened to chop off his head. It was discovered that Arthur was still alive and on his way home, but Mordred was still successful at persuading most of the English to take his side against Arthur, by promising them peace and joy.
When Arthur returned, on May 10th, Mordred failed to stop him landing at Dover, though in the battle (Arthur’s ninth) the still-weak Gawaine was fatally struck (yet again) on his recent head-wound, surviving just long enough to write to Launcelot, apologising and asking him to return to help Arthur. He died at noon and Arthur interred him in the crypt at Dover Castle (where it appears his marred skull was still on display in the late fifteenth century).
Arthur pursued Mordred and beat him again at Barham Down (Arthur’s tenth battle), and Mordred fled to Canterbury to strengthen his host from amongst Launcelot’s old supporters, and the counties of London, Kent, Sussex, Essex, Surrey, Suffolk and Norfolk.
The Trinity Sunday night, before they were due to battle again near the seaside outside Salisbury, Gawaine, flanked by all the lovely ladies he had ever saved, appeared to Arthur as he dozed, and told him to delay the battle for a month, while Launcelot came to his rescue. Arthur sent the two brothers, Sir Lucan the Butler and Sir Bedivere, to offer Mordred Cornwall and Kent in return for a delay, and the following morning both parties met to agree the deal.
As Arthur and Mordred (each flanked by an honour guard of fourteen) met to bargain on the field between their hosts, an adder bit a knight on the foot, and as he drew his sword to kill it, the worst was assumed, and soon the Battle of Salisbury (Arthur’s eleventh and last) had accidentally started.
By evening, the field was strewn with a hundred thousand dead and very few living, other than Arthur, the badly injured brothers Lucan and Bedivere, and Mordred, but at last Arthur had his recreant son in his sights, and taking his spear from Lucan, he went to kill him. He quickly impaled Mordred on his spear, but his son pulled himself along the length of it and as he died he struck Arthur a lethal blow to the head.
Looters had by now appeared on the darkening battlefield, and when the two brothers Lucan and Bedivere tried to move the dying Arthur to safety, Lucan’s guts fell out and he died.
Realising that his own end was near, Arthur commanded Bedivere to throw Excalibur into a nearby lake, but out of regard for the unique value of the sword, Bedivere twice hid it and lied, but could only say that he had seen it sink, so Arthur was not fooled. Driven by Arthur’s curses, at last Bedivere did as he was told, and thus could report back how he had seen a hand rise from the water, catch it, shake it thrice, brandish it, then pull it down.
Then he carried Arthur to that same water side, where he was put aboard a mysterious barge. On it were three queens: Queen Morgan le Fay, the Queen of Northgalis, and the Queen of the Wastelands, and also many damosels including Nimue, all in black hoods.
In spite of their ancient and bitter rivalry, Morgan rested Arthur’s head in her lap and said “Ah, dear brother, why have ye tarried so long from me? Alas, this wound on your head hath caught over-much cold.”
As the shrieking women bore him away across the waters, Arthur told Bedivere that he was being taken to the Vale of Avelion to be healed, and bid him farewell.
Alone and distraught, Bedivere wandered through the night, and by next morning had somehow arrived in Glastonbury, over 50 miles from Salisbury, where he found the old Bishop of Canterbury, now a hermit, standing vigil over a body which he said had been delivered at midnight by three queens, and which he believed was Arthur, but he couldn’t be sure. But just in case it was, Bedivere moved in with him to fast and pray over it.
Having received Gawaine’s note, Launcelot and his host finally landed at Dover, where he wept over Gawaine’s tomb. He then left his host behind and travelled alone to Almesbury to meet Guenevere, who had become a nun there upon hearing of Arthur’s death. After a very tearful scene, she told him to go back to France, and they parted for the very last time.
Launcelot then went to Glastonbury (another thirty miles), where he heard the full story from Bedivere, and he became a monk himself, joined within the year by Bors and seven of his knights who had become bored at Dover and had come looking for him (except Lionel, who had died in a skirmish in London on the search, and Ector, who was still looking for his brother elsewhere.)
Six years later, he had a vision, and he and the rest of the ageing knights rode feebly for two days to Almesbury to collect Guenevere’s corpse (she had died half an hour before they arrived), which they brought back to Glastonbury and buried next to Arthur (the main suspect). Launcelot pined from grief and guilt and was himself dead six weeks later. His body was taken to Joyous Gard for burial, where his brother Ector appeared just in time, after having wandered Britain for seven years looking for him.
After a prolonged fifteen-day wake, Sir Launcelot was finally buried, and the company returned to Glastonbury for a month.
Sir Constantine, son of Sir Cador of Cornwall, was made the new king of England, and ruled well. He sent the old hermit back to Canterbury to be Bishop again, and Bedivere went with him and remained a hermit too.
King Constantine also asked Ector, Bors, and the other French knights to stay with him, but they returned to their lands in France and became monks, except for Ector, Bors, Blamore, and Bloeberis, who headed East to fight the Turks in the Holy Land.