Englyn(n)ion y Beddau/Bedeu (Stanzas/Verses of The Graves), or Beddau Milwyr Ynys Prydain (Graves of Warriors of Isle of Britain);
primarily from Llyfr Du Cærfyrddin (Black Book of Cærmarthen) Tenth/Eleventh/Thirteenth Century AD, Llyfr Coch Hergest (Red Book of Hergest) c AD 1400, and Llyfr Gwyn Rhydderch (White Book of Rhydderch) c AD 1325:

‘Englynion y Beddau (Llyfr Ddu Caerfyrddin XIX)’ at Academy for Ancient Texts. Ancient Texts Library.
https://www.ancienttexts.org/library/celtic/ctexts/bbc19w.html

‘The Stanzas of the Graves (The Black Book of Carmarthen XIX)’ at Academy for Ancient Texts. Ancient Texts Library.
https://www.ancienttexts.org/library/celtic/ctexts/bbc19.html