Where did the story of King Arthur first appear

Arthur first appears in Welsh literature. In a surviving early Welsh poem, The Gododdin (ca. 594 AD), the poet Aneirin (ca. 535-600 AD) writes of one of his subjects that "he fed black ravens on the ramparts, although he was no Arthur." However, this poem (as it currently exists) is full of interpolations, and it is not possible to decide if this passage is an interpolation from a later period. The following poems attributed to Taliesin are possibly from a similarly early date: The Chair of the Sovereign, which refers to "Arthur the Blessed"; Preiddeu Annwn ("The Treasures of Annwn"), mentions "the valour of Arthur" and states "we went with Arthur in his splendid labours"; and the poem Journey to Deganwy, which contains the passage "as at the battle of Badon with Arthur, chief giver of feasts, with his tall blades red from the battle which all men remember."