Llacheu is Arthur’s son in the earliest Welsh legends and the only of the three sons mentioned in Welsh tradition–Amr and Gwydre being the others–who made it into the later continental romances.
A strange tradition also exists that Llacheu may have been killed by Sir Kay. The following is a passage discussing this possibility from my book King Arthur’s Children: A Study in Fiction and Tradition, which contains a much longer discussion of Llacheu and is available at www.ChildrenofArthur.com
The French, however, when they learned the Welsh tradition that Arthur had a son, either ignored or did not know his true place in the legends and simply let their imaginations run wild (Bruce, “Arthuriana,” 184). It may be Geoffrey of Monmouth’s responsibility that Arthur’s sons disappeared from later versions of the legends; The History of the Kings of Britain was so popular that it firmly placed a structure on the way the tale would be told from then on, and since Geoffrey did not give Arthur any sons, his successors avoided creating sons for Arthur. And if writers had added sons to the legend, they would have had to come up with explanations for why these sons did not succeed their father. However, the fact that Llacheu does appear in romances written after Geoffrey of Monmouth is a clear indication that the French writers had some knowledge (however limited it may have been) of the Welsh traditions from Breton traditions, independent of Geoffrey of Monmouth (Bruce, “Arthuriana,” 179).
One aspect of Llacheu’s story the French appear to have added was his illegitimacy through his mother Lisanor, a woman who is probably completely fictional. More importantly, the French and their followers created a whole new death story for Llacheu that has come down to us in two different, but closely related versions.
In all of these versions, Llacheu is slain by Sir Kay. Several scholars have suggested that the source for Kay’s murdering Llacheu was Llacheu and Kay’s names being mentioned together in The Black Book of Caermarthen as follows:
Unless it were God who accomplished it,
Cai’s death were unattainable.
Cai the fair and Llachau,
they performed battles
before the pain of blue spears [ended the conflict].
(Bromwich, Arthur of the Welsh, 43)
The two warriors may have fallen together in battle, but Bruce and other scholars believe it is evident from the way the names are coupled that Kay was not Llacheu’s slayer in Welsh tradition (Bruce, “Arthuriana,” 184). Kay seems to have been accused of murdering Llacheu, much as the mention of Arthur and Mordred falling together at Camlann in the Annales Cambriae may have been interpreted as Mordred revolting against Arthur.
In his Studies in the Arthurian Legend, John Rhys gives one version of Llacheu’s murder which he takes from the second part of the Welsh version of the Grail, the Seint Greal. In this version, Llacheu seeks adventure and fights a giant named Logrin, who has proven himself one of King Arthur’s cruelest foes and allows no one to live in the same country with him. Llacheu succeeds in killing the giant and then lies down on the giant’s body and falls asleep. Kay then rides up, discovers this strange sight, and beheads Llacheu and the giant. He then returns to court, claiming he slew the giant. The court makes much of him, but soon his treachery is known and hostility grows between Arthur and Kay, causing Kay to flee to his own castle (61).
Another version of this story occurs in the Perlesvaus, a French work of the early thirteenth century. Here everything occurs as in the last story up to where Kay kills Llacheu. This time, Kay cuts off both the giant and Llacheu’s heads and brings Llacheu’s body, along with the giant’s head, back to court, claiming he killed the giant who had killed Llacheu. Later a damsel comes to court with a coffer containing Llacheu’s head, and she tells the story of his death. Guinevere recognizes the head as having belonged to her son from a scar that is on it; the sight of it causes her to die of grief (Bruce, “Arthuriana,” 182).
Caitlin and John Matthews, in the The Arthurian Book of Days (1990), give a version of the tale that makes Kay look more like a victim than a murderer; however, they do not give their source. It appears in the entry for March 15, as follows:
Arthur sat in solemn justice to hear the defence of his foster brother. Kay stammered his sorry tale: “Upon my last quest I encountered a giant who made me play a beheading game. I knew the way of it, I thought, since Gawain’s contest those many Christmases ago. Instead of himself, the giant sent forth against me a knight who acquitted himself nobly, but I overcame him and struck off his head. It was not till the helmet was off that I saw it was Loholt, and that I had been tricked into treachery. Until the ending of my life, I repent that stroke.” (45-6)
Here Morgain interrupts to relate that the giant is the brother of King Arthur’s enemy, King Rhitta, and that this event is the sorrow she foretold.
And since Kay had been shamed by such a trick, Arthur forgave him before all, though Guinevere was less forgiving. (45-6)
Since Caitlin and John Matthews do not give a source for this version of Llachue’s death, it seems logical to assume that they were merely rewriting the tale as it appeared in the Perlesvaus since Kay’s motive for murdering Llacheu is not expressed in that work; furthermore, they also added in the detail of Gawain playing a beheading game, an event that occurs in Sir Gawain and the Green Knight, a late fourteenth century Middle English work; the Matthews reinterpret the tale by giving Kay a form of motivation for killing Llacheu, which makes Llacheu’s murder more plausible.
In the two earlier versions, Kay is clearly an intended murderer, rather than a victim of someone else’s evil deeds. This depiction of Kay is surprising since in the Welsh tales he usually appears as the greatest, or at least one of the greatest of Arthur’s warriors, plus his loyal subject, friend, and foster-brother. However, Kay is sometimes depicted as being touchy toward Arthur as at the end of “Culwch and Olwen,” where a hint of some discord between Arthur and Kay exists, although it seems unlikely that in the Welsh tradition Kay would have stooped to murdering Arthur’s son; therefore, the story of Llacheu’s murder is probably of continental origin.