Gildas/Gyldus [III]. De Excidio et Conquestu Britanniæ (On Ruin and Conquest of Britain). Sixth Century AD (after AD 559 (572/573), possibly as late as AD 598):

Giles, John Allen, tr. The Project Gutenberg eBook of On the Ruin of Britain.
Bert Olton, prod. Release date: November 1, 1999 [eBook #1949], Most recently updated: December 31, 2020.
http://www.gutenberg.org/cache/epub/1949/pg1949.txt

Williams, Hugh, tr and ed. Gildae: De Excidio Britanniae, fragmenta, liber de paenitentia, accedit et Lorica Gildae.
Gildas: The Ruin of Britain, Fragments from lost letters, The penitential, together with The Lorica of Gildas.
London: David Nutt, 1899.
https://ia800201.us.archive.org/14/items/gildaedeexcidiob00gilduoft/gildaedeexcidiob00gilduoft.pdf