According to Welsh legend, the Emperor Magnus Maximus, known as Macsen Wledig (the Imperator), was a widowed senator living in Rome. Being a minor member of the Constantinian Imperial family, he felt it unjust that the Empire was ruled by the Emperors, Gratian & Valentinian, but there was little he could do about it.
In about 365, Maximus was out hunting one day when he rested beneath a tree and fell asleep. He had a long dream about a palace far away. He entered the palace and encountered an ageing King and two young men playing chess. Turning, his eyes met the most beautiful woman he could ever have imagined, sitting on a golden throne. On waking, Maximus immediately sought out a local oracle who urged him to search out this beautiful maiden. So messengers were sent out across the Empire but, dispite exhaustive searches, all returned empty handed. There was no sign of Maximus' beauty.
Meanwhile, at the edge of the Empire, High-King Eudaf Hen of Britain was getting very old. He decided it was time to appoint his official heir to the British Kingdom. His nephew, Conan Meriadoc, was perhaps the most obvious choice, though the King's direct heir was his only daughter, Elen. Eudaf's chief advisor, Caradoc, the King of Dumnonia, advocated strengthening Roman links by marrying Elen to a man with Imperial connections. The two could then inherit the Kingdom together. He knew of such a steady young man in Rome who would make an ideal husband. Eudaf was intrigued. So had Caradoc send his son, Mauric, to seek this Roman out.
Mauric arrived in Rome at the house of Magnus Maximus, just as he had received the unfortunate news that his dream girl could not be found. Glad of the distraction and persuaded by Mauric's suggestion that he might find support in Britain for his Imperial claims, Maximus gladly agreed to return with him. Comes Theodosius' historical expedition to Britain in order to quell barbarian risings actually brought Magnus Maximus to these shores in 368. Legend tells how the arrival on the island of a large army of men caused quite a stir and, not realising who it was, Eudaf sent Conan with an army to disperse them. Fortunately, Mauric persuaded all of their good intentions and Maximus was able to ride off to Eudaf's court at Carnarfon (Caer-yn-Arfon alias Caer-Segeint).
Upon being introduced to everyone, Maximus was astounded to find that Eudaf was the old man in his dream and Conan, one of the chess-players (some say the other was his son, Cadfan). He was then overjoyed to find that Eudaf's daughter, Elen, was his dream-girl. The two fell in love immediately and were married with great pomp and ceremony.
Eudaf died soon afterward, and Maximus and Elen inherited his Kingdom. Conan was extremely annoyed and rode north to gather an army of Picts & Scots to overthrow them. However, Maximus defeated him and, being magnanimous in victory, the two made peace. Conan became Maximus' dearest friend and also his magister militum.
At this point, we return to more historic details. News reached Britain that Maximus' relative, Theodosius had been elevated to the Eastern Imperial throne. Incensed, Maximus invaded the Western Empire, in 383, along with his son Victorius and 'Prince' Conan. They withdrew troops from Carnarfon (Caer-Segeint) & elsewhere in Britain and his men quickly proclaimed Maximus as Emperor. His armies marched across the continent, establishing his rule as they went. Conan eventually killed the Western Emperor, Gratian, in battle (being given Brittany as a reward), and Maximus became sole ruler of the West.
Maximus set up his capital at Trier and ruled well over Britain, Gaul & Spain for four years. He was baptised a Christian, and was recognised as Emperor by Theodosius who was occupied with his own troubles elsewhere. Eventually however, Maximus was forced to make a move against Gratian's younger brother, Valentinian, the Southern Emperor, who threatened his rule from Rome. He invaded Italy, took Milan and for a whole year besieged Rome, before Conan arrived once more and finished the job. Unfortunately though, Valentinian escaped. He soon returned, backed up by the Roman Emperor of the East, Theodsoius. Maximus' forces were twice defeated at Illyricum, before he was finally killed, with his son, at Aquileia.
Sources
Geoffrey Ashe (1990) Mythology of the British Isles.
Gildas Badonicus (c.540) The Ruin of Britain.
Peter C. Bartrum (1993) A Welsh Classical Dictionary.
A.H.M. Jones (1964) The Later Roman Empire 284-602: A Social, Economic, and Administrative Survey.
Geoffrey of Monmouth (1136) The History of the Kings of Britain.
Nennius (c.829) The History of the Britons.
The Red Book of Hergest (14th c.) The Dream of Macsen Wledig .
William Smith & Henry Wace (1877) The Dictionary of Christian Biography.
The White Book of Rhydderch (14th c.) The Dream of Macsen Wledig.