What was happening was the conquest of (most of) Britain by the Saxons and Angles. England ("Angleland") came into existence through this invasion. Arthur was originally a mythical (possibly - but not certainly - based on a real person) Brythonic king who temporarily drove back the Saxons. But most of the stories associated with him are either later innovations or old but originally unrelated myths that were reworked to benefit from the popularity of Arthurian literature in the High Middle Ages. Arthur was portrayed as a late medieval knight which - if he existed at all - he surely was not. There's little written history (or any written material at all) from Western Europe in that period (which is why it's called a Dark Age). What we do have was mostly written by monks and mainly concerned with religious subjects like the lives of saints.