Question: Ambrosius Aurelianus as a war leader of the Romano-British who won an important battle in the 5th century, according to a 6th-century British monk best known for his scathing religious polemic which recounts the history of the Britons before and during what? 
Sentence (from "Gildas"): In his later life, he emigrated to Brittany where he founded a monastery known as St. Gildas de Rhuys.
Relevant evidence: No