Central Middle Age texts
A number of texts, probably in the range of the 10th to 12th centuries, refer to Arthurian stories within their body without being Arthurian-esque themselves. An example of this is in the Book of Taliesin: the name Arthur appears a handful of times in the book, with the most notable reference being in poem Preiddeu Annwfn - The Spoils of Annwfn. The poem is written as if in the voice of Taliesin, in which the narrator alludes to an experience about a journey accompanying Arthur to Annwfn, a Celtic otherworld.
There are a few Arthurian references in the Englynion y Beddau, Stanzas of the Graves - poetry that records the graves of archaic Welsh battle heroes. The most significant reference to Arthur mentions his grave, or rather lack of it; his grave is impossible to find, drawing on the idea that Arthur is in fact not dead. The englynion exist in the Black Book of Carmarthen though it is probable that they are older than the manuscript's 13th century date.
The popular and romanticised image of Arthur with his knights and Merlin derives from the hugely influential Historia Regum Britanniae (c.1138) written by Geoffrey of Monmouth.
Geoffrey introduced many of the aspects of the Arthurian legend as we know it today in his work; his conception at Tintagel, naming his father as Uther Pendragon, his reign as a king, the final battle at Camlan and his transferral to the island of Avalon following his mortal wounding. It is likely that his Historia was a catalyst in the growth of literary popularity of Arthurian romances, which were to remain popular throughout Europe for centuries afterwards.
Arthur is also mentioned a number of times in the Welsh Triads, or Trioedd Ynys Prydein, which mention his royal palace and his court. These were texts in medieval manuscripts that reflected Welsh history, folklore and mythology. They are regarded by scholars to have been mnemonics for poets to recall narrative material.
The earliest surviving text of the Triads is in the manuscript Peniarth 16, now kept at the National Library of Wales in Aberystwyth, though a more extensive version exists in the Red Book of Hergest.