Armes Prydein (Vawr) (Prophecy of (Great) Britain), from Llyfr Taliessin (Book of Taliesin). c AD 930 (early/mid Tenth Century AD):

‘Armes Prydein Vawr: Llyfr Taliesin VI’ at Academy for Ancient Texts. Ancient Texts Library.
https://www.ancienttexts.org/library/celtic/ctexts/t06w.html

‘Armes Prydein Vawr (The Prophecy of Prydein the Great): Book of Taliesin VI, from The Four Ancient Books of Wales’ at Academy for Ancient Texts. Ancient Texts Library.
https://www.ancienttexts.org/library/celtic/ctexts/t06.html