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Gracianus Municeps was a legendary King of the Britons, according to Geoffrey of Monmouth's Historia regum Britanniae (Latin: History of the Kings of Britain), a largely fictional account of British history. After the death of Roman Emperor Magnus Maximus, Gracianus seized the throne of Britain upon receiving word of Maximus's demise, by whose orders he had been sent to defend the attacked island while Maximus was campaigning on the continent.
Gracianus served under Maximus during his campaigns in Rome and Germany, and was sent to Britain to defeat Wanius and Melga, the kings of the Picts and Huns respectively. He defeated the armies of both kings immediately upon arrival, ejecting them to Ireland. Soon after, word came that Maximus had died at the hands of either a supporter of the late Roman Emperor Gratian or by one of Gracianus Municeps' own followers. Despite mention previously made by Geoffrey of Monmouth of Dionotus, regent in Maximus' absence and king of Cornwall, Gracianus seized the crown of Britain and began a reign of terror throughout the island but soon certain plebs banded together and assassinated him. This led to a period of instability when news of his demise reached Britain's enemies, but he was eventually succeeded by Constantine II of Britain, the brother of King Aldroenus of Brittany.
Historically, the predecessor to Constantine was Gratian on whom Geoffrey's tale was probably based. The Venerable Bede refers to this Gratian as Municeps in his Historia ecclesiastica gentis Anglorum (Latin: Ecclesiastic History of the English People) in Chapter XI of this work and the epithet is seemingly there to distinguish this Gratian from the earlier Gratian killed by the Usurper Magnus Maximus.