Lyonesse was one of the last parts of the western empire to be conquered by barbarians, having resisted the Franks until 486 under the last magister militum per Gallias, Syagrius. After conquest it formed the Neustrian division of the Frankish realm and was the base for further expansion south. In 640 Aquitania was lost and King Dagobert was killed at the Battle of Castras, resulting in the disintegration of Frankia and the effective independence of Neustria under Dagobert's second son, Clovis II.
In 652 Neustria was invaded by the newly resurgent Visigoths, using rebellions of the Armorican Britons as a pretext. Heavily outnumbered, Clovis fled to his brother's court in Austrasia, where he plotted to regain his throne for the rest of his life. In Neustria the Visigoths installed one Caesarius, a leading member of the local Gallo-Roman aristocracy, as a client king, while Armorica became an independent ally.
In the 750s Neustria went to war against Pepin, King of the Franks, who had created a standing army and used it to invade and subjugate many of the surrounding tribes. In 753 the Neustrians were decisively beaten at the Battle of Compiègn, and the country was only saved from destruction by the overdue arrival of ten thousand Goths. However, in 768 Pepin's even more capable son Charles became king and, after completing the subjugation of the Saxons and Bavarians, in 775 he succeeded in defeating the Goths and conquering Neustria.
Over the next twenty years Charles campaigned all over Europe, eventually also incorporating Burgundy, Italy, Armorica and Carinthia into his domains. Finally, in 798 he led a daring raid deep into Gothic territory, capturing by surprise their capital at Barcelona and taking as part of the spoils the imperial regalia of the Western Empire, which had been in Gothic hands ever since the days of Emperor Wamba the Great.
In 800, Charles brought the imperial regalia with him on a pilgrimage to Rome, where on Christmas Day he was crowned Western Emperor by Pope-King Leo III. This event is commonly held as the beginning of the Holy Roman Empire.
After the death of Charles' son Louis his empire was divided. The western part, which evolved into Lyonesse, eventually became a separate kingdom while the eastern part, which was still little more than a loose confederation of tribes, became the HRE of later centuries. Aquitaine was reconquered by the Goths within a generation, and Burgundy and Italy began to set out on their own paths.
At the same time as Viking fleets were attacking Albion, they were also raiding the shores and riverbanks of the continental mainland, including Lyonesse. In 911 one of their lesser leaders, Hrolfr Rognvaldsson, sailed up the Seine, attacked Paris, and then laid siege to the city of Chartres.
The Bishop of Chartres called for help from King Charles the Simple of Lyonesse. However, having lost the allegiance of so many lords of Aquitaine and Burgundy, the king was unable to gather a sizeable army. At the Battle of Chartres on 20 July, the Franks barely managed to fight the Vikings to a standstill, and Charles was forced to sue for peace.
The Treaty of Saint-Clair-sur-Epte granted Hrolfr a vast swath of land along the coast, gave him Charles' daughter Gisela, and recognised him as Charles' sole heir. Hrolfr changed his name to the Lyonnaise form Rollo, and after Charles had died in 929 during a rebellion of the nobles, Rollo crushed the revolt and had himself crowned King of Lyonesse.
Rollo named his dynasty the Normans, a corruption of "Norsemen". The Norman dynasty lasted until 1135, after which it was replaced by the Angevins.
In 1328 the throne of Lyonesse was inherited by Richard IV, nephew of Henri V, the last Angevin king. Richard was already King of Prydain and Duke of Arvor, and his acquisition of Lyonesse meant that he controlled all of the lands on both sides of the Gallic Channel, making him one of the most powerful rulers in Europe.
The three states remained united until 1413, when Guillaume IV divided his lands between his two sons. The elder, Aneirin, received Prydain and its subjects within the Albic Isles, while the younger, Ambrose, received Arvor and Lyonesse. In time, Arvor was absorbed into Lyonesse so that today it retains little of its separate identity.
Fátime, granddaughter of Ambrose II, was the last Jersiais monarch of Lyonesse, though the dynasty continued elsewhere. After her death in 1461 the throne passed to her husband Louis, Comte de Champagne.