We are then told by Geoffrey that not only did Arthur conquer Britain but also that he conceived the idea of conquering Europe, later striding forth making great headway in Denmark, Norway and Gaul. Arthur is seen to return back to his home in Wales as a victor and there followed a period of twelve years of peace. He was believed, in Welsh legend, and by many scholars, to have returned to home to a place known as 'Caerleon' (See Caerleon). Here Arthur is said to have had a great court, a match for any in Rome, where trading and tournaments took place. Arthur is depicted as planning to conquer Rome when he was subject to the treachery of 'Mordred', (See Mordred), and also of 'Guinevere' (See Guinevere), which leads Arthur to engage in the fatal 'Battle of Camlan' (See Battle of Camlan), with his nephew Mordred, when Mordred was slain and Arthur was mortally wounded. This is when Arthur is believed to have been taken to the Isle of Avalon to have his injuries attended to.

With the disappearance of Arthur, to Avalon, it seems that Britain broke into civil war. In Geoffrey of Monmouth's book, we are not told Arthur dies, in accordance with Arthurian legend, but in fact Geoffrey strongly hints that one day Arthur would return to save the Britons. It was this ending that gave the Norman's their biggest headache on how they could anglicise and reconcile Arthur's heritage and reappearance into Norman history if there was any chance he might return, yet at the same time, the possibility of Arthur returning provided the Normans with the perfect opportunity to gain support for ousting the Saxons and removing Britain out of their control.