"De Excidio et Conquestu Britanniae" is a historical work written by the monk Gildas in the 6th century. It provides a detailed account of the decline of Roman Britain and the subsequent invasions by the Saxons. Gildas critiques the moral failings of the British leaders and emphasizes the need for repentance and reform. The text serves as one of the earliest sources on post-Roman Britain, offering insights into the social and political landscape of the time. It highlights the struggles faced by the Britons after the withdrawal of Roman authority and the impact of external threats on their society.