Gracianus Municeps (also known as Gratianus) 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.
[1] Gracianus was a freedman, who had 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.
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.
Historically, the predecessor to Constantine was Gratian on whom Geoffrey's tale was probably based.