Arthur was killed by a giant cat.*
Arthur killed the cat.
Arthur didn’t fight the cat. Kay did.
Kay and Bedivere use salmon as taxis.
Lucan is half giant, half lion. (This Lucan, Lucano in the original Italian, is evil and not related to Bedivere).
King Arthur raided the land of the dead.
The human knight Caradoc Briefbras has three half siblings: a dog, a horse, and a pig.
A large portion of Arthur’s troops was killed a while before Badon by his nephew’s attack ravens in self-defense. Arthur and said nephew were playing chess at the time and neither did much to stop it. [Edit: before Badon, not Camlann, which has apparently already happened despite Arthur and Mordred being alive]
Merlin retired peacefully and went to live in the countryside with his also-magic sister Ganieda, Taliesin, and another of their friends. [Edited]
Wherever Arthur walks, plants die. They don’t grow back for years.
Arthur had a spunky (half?) brother who died in battle after making a mysterious oath.
Dagonet is more or less able to run the kingdom when Arthur is gone. His biggest error is overspending on mercenaries.
Guinevere has an evil almost identical twin half-sister.
Hector beat up all the best knights except for Galahad while possessed by a demon.
Gawain plays tennis.
Gawain has used a chessboard as a weapon.
Near the start of his reign, Arthur left Lot in charge of the kingdom and went on a quest with a sassy parrot.
Gawain or Galahad succeeded Arthur as king.**
*Whether or not this is canon anywhere is a somewhat meta matter. André de Coutance complains that the story that Chapalu/Cath Palug killed King Arthur and conquered England is a slanderous lie while also implying it's widely circulated. He's saying that it's canon in other places and also that it's wrong. As far as I know, no other text mentions a tradition where the cat kills the king.
**Not in different texts--Bhalbhuaidh is either Irish Gawain or Irish Galahad.
Lucan's doing well post camlann i guess
Arthur: Would you die for me?
Lucan: Of course, my liege. If I had to.
Arthur: Would you die for me?
Bedivere: As the Marshal of Camelot and a man of honor, it is my duty to do whatever is required of me by the throne.
Arthur: Would you die for me?
Griflet: No. That would be stupid. I would hold you in my arms as you died, then burn all your possessions.
Arthur: Out of grief?
Griflet: Yeah. Grief.
“Scared at last the maiden took refuge by the tripods; she drew near to the vast chasm and there stayed; and her bosom for the first time drew in the divine power, which the inspiration of the rock, still active after so many centuries, forced upon her. At last Apollo mastered the breast of the Delphian priestess; as fully as ever in the past, he forced his way into her body, driving out her former thoughts, and bidding her human nature to come forth and leave her heart at his disposal. Frantic she careers about the cave, with her neck under possession; the fillets and garlands of Apollo, dislodged by her bristling hair, she whirls with tossing head through the void spaces of the temple; she scatters the tripods that impede her random course; she boils over with fierce fire, while enduring the wrath of Phoebus. Nor does he ply the whip and goad alone, and dart flame into her vitals: she has to bear the curb as well, and is not permitted to reveal as much as she is suffered to know. All time is gathered up together: all the centuries crowd her breast and torture it; the endless chain of events is revealed; all the future struggles to the light; destiny contends with destiny, seeking to be uttered. The creation of the world and its destruction, the compass of the Ocean and the sum of the sands—all these are before her.”
— Lucan, Pharsalia 5:161ff, tr. J. D. Duff.