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.
They call me "little man," "King Arthur's fool,"
And "simpleton," those lackeys at the court,
But this fool's mother had the Second Sight,
And sometimes when I caper for the king
I see more than Taliesin the bard
And Merlin the enchanter can, combined.
I stand before the dais, juggling:
The red balls first, then yellow, green and blue,
And when I add the gold and silver spheres,
The oval blur between my hands takes form.
A glowing, rainbow mirrow it becomes
Through which I see the king an older man.
His beard is shot with grey. Astride his horse
He sits up straighter than he would on land
When all the kingdom's cares, some awful guilt,
And the death of all his dreams lie on his back.
I see two rows of soldiers and a snake,
A sword unsheathed to kill it, turned on him--
I drop the balls and stammer out some jest,
A wish for pardon, while the courtiers roar.
He does not laugh. He sees my face go grey
With terror. Arthur thinks I fear his wrath.
He hands me the gold ball, rolled to his feet,
Says, "Dagonet, all people make mistakes."
He glances at his wife; she looks away.
Fool I may be, but even I can tell
There's something wrong when Guinevere looks down
Among the milling courtiers at one knight,
The tallest, bravest, handsomest in spurs:
At Lancelot, who never makes mistakes.
I scramble for the balls. He looks at me,
Then looks away, and shrugs his lion's mane.
Dismiss me as a fool, Sir Lancelot.
Better a fool in small things all my life
Than a great lord who, with one folly alone,
Casts all he loves to ruin at life's end.