What is "gildas"

Gildas (c. 500–570) (also known as "Gildas the Wise" or Gildas Sapiens) was a 6th-century British monk best known for his scathing religious polemic De Excidio et Conquestu Britanniae, which recounts the history of the Britons before and during the coming of the Saxons. He is one of the best-documented figures of the Christian church in the British Isles during the sub-Roman period, and was renowned for his Biblical knowledge and literary style. In his later life, he emigrated to Brittany where he founded a monastery known as St. Gildas de Rhuys.