King Arthur is not completely shrouded in mystery. Although everything about him is controversial, there are a few facts about Arthur in the Historia Britonum and the Annales Cambriae that are likely to be more or less accurate. And we don't know a lot about any persons that lived in Britain before and after him during the mysterious period of about 400 to 600 AD. Everyone else mentioned in various sources is just as dimly and vaguely understood as Arthur.