... First for those in Welsh literature, which, as will be observed, are very meager.  The earliest, no doubt, are the passages in the Black Book of Caermarthen, in which we have merely obscure allusions to the death of this son of Arthur, Llachau, as he is called.  I quote from the translation in W. F. Skene's The Four Ancient Books of Wales, 2 vols., Edinburgh, 1868.  Vol. I, p. 263, of this work we find in the "Poems referring to Arthur the Guledig" the line:  "Unmerited was the death of Cai the fair and Llachau" (= Black Book of C., XXXI).  Ibid., p. 295, in the "Poems referring to Gwyddno and Gwynn ap Nudd" occurs the stanza (from Black Book, XXXIII):

I have been where Llachau was slain,
The son of Arthur, extolled in songs,
When the ravens screamed over blood.

Llachau is here simply one of a number of dead heroes mentioned in successive stanzas.
    Furthermore, in the Dream of Rhonabwy, J. Loth's Mabinogion (2 vols., Paris, 1889), I, 312, "Llacheu," son of Arthur, is enumerated among the king's counsellors, and in a Triad (see ibid., II, 27) he is called one of the three deivniawc¹⁰ of the isle of Prydein. 
    The earliest mention of the character in the French romances is that in Chrétien's Erec, l. 1732.  He is there named in the famous list of Round Table heroes:

Et uns vaslez de grant vertu
Loholz li fiz le roi Artu.¹¹

    ¹⁰ J. Loth explains, "probablement inventeurs, qui devinent la nature des choses."

(The Romantic Review, Todd, v.3, 1912).