Constantius I (Latin: Marcus Flavius Valerius Constantius Herculius Augustus; 31 March c. 250 – 25 July 306), commonly known as Constantius Chlorus (Greek: Κωνστάντιος Χλωρός, Kōnstantios Khlōrós, literally "Constantius the Pale"), was Caesar, a form of Roman co-emperor, from 293 to 306. He was the father of Constantine the Great and founder of the Constantinian dynasty. 
Born in Dardania, Constantius was the son of Eutropius, whom the Historia Augusta claimed to be a nobleman from northern Dardania, in the province of Moesia Superior. 
Conventional accounts give his mother's name as Claudia, a niece of the emperors Claudius II and Quintillus Modern historians suspect this maternal connection to be a genealogical fabrication created by his son Constantine I, and that his family were of humble origins
He was first married to Helena but required to divorce her in 293. 
He married secondly, in 293 and until 306, Theodora (Flavia Maximiana Theodora)
Governor of Dalmatia, appointed Caesar to rule Gaul and Britain March 1, 293.
Emperor of the Western Roman Empire, father of Constantine the Great, founder of the Constantinian dynasty.
Constantius I became Emperor of Rome in May 305, and in right of his wife, King of England. He was born in 242 and died at Eboracum (present day York, England) on July 25, 306. On becoming "Caesar," he was required by Diocletian to put aside *Helena and to take Maximian's stepdaughter, Theodora, as his wife. From the first union, *Helena and *Constantius I had an illegitimate son, *Constantine the Great. He married (2) Theodora, daughter of Maximinus, Roman Emperor.
Constantius I, called Constantius Chlorus, Roman emperor (305-06). He was a general and administrator under Emperor Maximian, who adopted him and gave him the government of Gaul and the rank of Caesar in 293. When his co emperors, Maximian and Diocletian, abdicated in 305, Constantius became emperor in the West and prepared to conquer the Picts of Scotland.He died during the campaign, after proclaiming his son, Constantine the Great, his successor as emperor.
Constantius died suddenly in Eburacum (York) in 306. As he was dying, Constantius recommended his son to the army as his successor; consequently Constantine was declared emperor by the legions at York. His death sparked the collapse of the tetrarchic system of government inaugurated by the Emperor Diocletian.
Born in Dardania, the Historia Augusta claimed Constantius was the son of Eutropius, a noble from northern Dardania, in the province of Moesia Superior (roughly today's Serbia), and Claudia, a niece of the emperors Claudius II and Quintillus.
Constantius' father, however, might have been the brother of Eutropia, wife of Maximian.
Gibbon stated that Claudia was the niece of Claudius II (Marcus Aurelius Flavius Claudius Gothicus).
Encyclopedia Brittanica maintains Constantius I's descent from Claudius Gothicus was a fiction.
Wikipedia states that "Constantius was ... remembered in medieval Welsh legend, which confused his family with that of Magnus Maximus, also said to wed a Saint Elen and sired a son named Constantine while in Britain.
Henry of Huntingdon, Historia Anglorum
Henry of Huntingdon's, "History of the English" identified Constantius's wife Helen as British.
Huntingdon's history is apparently the origin of a pedigree appearing in Richard Morgan's 1861 book, St. Paul in Britain, discussed in more detail at Space: From Beli to Byzantine.
1136 Geoffrey of Monmouth
Geoffrey of Monmouth repeated the claim in his 1136 "History of the Kings of Britain". Geoffrey related that Constantius was sent to Britain by the Senate after Asclepiodotus (here a British king) was overthrown by Coel of Colchester. Coel submitted to Constantius and agreed to pay tribute to Rome, but died only eight days later. Constantius married Coel's daughter Helena and became king of Britain. He and Helena had a son, Constantine, who succeeded to the throne of Britain when his father died at York (Eboricum) eleven years later. These accounts have no historical validity: Constantius had divorced Helena before he went to Britain.
Similarly, the "History of the Britons," traditionally ascribed to Nennius mentions that the inscribed tomb of "Constantius the Emperor" was still present in the 9th century in Segontium (near present-day Caernarfon, Wales).
Ford credited the monument to Constantine, supposed son of Magnus Maximus and Elen, said to have ruled over the area prior to the Irish invasions." 
1 Constantius' full name: ILS , 630, 641-3, 648, 650-52; such variations as Marcus Flavius Valerius Constantius( ibid , 637), Valerius Constantius ( ibid. , 640), Gaius Valerius Constantius ( ibid , 649), and Gaius Fabius Constantius ( ibid. , 650a) appear on inscriptions.
Constantius' date of birth, homeland, and career: Michael DiMaio, Zonaras' Account of the Neo-Flavian Emperors: A Commentary , (Ph.D diss., University of Missouri-Columbia, 1977), 97-98, nn 11-14; Constantius as Maxiamianus' Praetorian prefect: T.D. Barnes, Constantine and Eusebius, , (Cambridge, 1980), 3, 7-8, New Empire of Diocletian and Constantine , (Cambridge, 1981), 36-37.
For a discussion of Constantius' marriage to Helen, see infra , n.2.
2 For a listing of the sources on the dating Constantius rise to the rank of Caesar, see A.H.M. Jones, J.R. Martindale, and J. Morris, the Prosopography of the Later Roman Empire , (Cambridge, 1971), s.v. "Fl. Val. Constantius 12," 1.227-228, and Dietmar Kienast, Römische Kaisertabelle , (Darmstadt, 1990), 276ff
The Caesarship of Constantius is discussed, for example, by O. Seeck, RE 4, s.v. "Constantius 1," col.1041.57ff, and Ramsey MacMullen, Constantine , (New York, 1969), 35ff.
Barnes has summarized the problems and the sources surrounding the marriages of Constantius to Helen and to Theodora ( New Empire ), 33, 36-37.
3 Constantius' campaigns are discussed by Barnes, Constantine and Eusebius , 15ff, and by Harold Mattingly and B.H. Warmington, OCD ,2s.v ."Constantius (Chlorus)," 281-282.
4 Lact., Mort. Pers. , 15.7
5 For a listing of sources which treat the acclamation of Constantius to the rank of Augustus, see Barnes, New Empire , 4ff.
6 For a discussion of the death of Constantius and the sources that treat it, see DiMaio, 96ff.
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