Magnus Maximus (383-88) was a Spaniard. His early career was influenced by his relationship with Theodosius the Elder.
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.
Eudaf died, and Maximus and Elen inherited his Kingdom. Cynan 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. Cynan became Maximus' dearest friend and also his magister militum.
Maximus' regime seemed to function smoothly. He ruled in Britain, Gaul, Spain, and Africa. He issued coinage, promulgated laws, and had an imperial bureaucracy. Maximus was a staunch Nicene Christian. He executed the Spanish heretic Priscillian sometime between 384-86, arousing the ire of Pope Siricius and other prominent members of the church. This was ostensibly over doctrinal issues, but was really a question of authority. In addition, he reorganized the provinces in Gaul, most notably creating Lugdunensis III and Lugdunensis Senonia, as well as giving his name to Maxima Sequania. His associate Gildo ruled Africa.
In 387, Maximus decided to invade Italy and displace Valentinian II once and for all. Valentinian fled to Theodosius in the east and requested help. Theodosius mustered an army and marched west. After several battles in Illyricum at Emona, Siscia, and Poetovio, Maximus was captured and executed at Aquileia on 28 August 388. Maximus was important for several reasons. The trend of generals holding de facto power in the west had begun. His relations with the church began the long battle over secular rulers having power over the church. In addition his use of barbarians was another foreshadowing of the conditions prevalent in the west during the fifth century. Finally, Maximus was the last really powerful emperor in the west; his defeat all but insured that the center of the Roman empire was to become Constantinople.
According to Geoffrey of Monmouth's fictional Historia regum Britanniae, basis for many English and Welsh legends, Magnus was king of the Britons following the death of Octavius and a nephew of King Coel through his brother Ioelinus. According to the Mabinogion tale The Dream of Macsen Wledig, Magnus Maximus took as his wife Helen or Elen, daughter of a chieftain based at Segontium (Caernarfon). This agrees with the story Geoffrey tells that Octavius, the king of the Britons, wanted to wed his daughter to a powerful half-Roman, half-Briton and give the kingship of Britain as a dowry to that husband.