Lucius Aelius Megas Abgar IX was a Syriac ruler of Osroene

Osroene

Osroene, also spelled Osrohene and Osrhoene and sometimes known by the name of its capital city, Edessa , was a historic Syriac kingdom located in Mesopotamia, which enjoyed semi-autonomy to complete independence from the years of 132 BC to AD 244.It was a Syriac-speaking kingdom.Osroene, or...

 from AD 177 to 212.

Andrew Louth in his "Who's Who in Eusebius" at the end of G. A. Williamson

G. A. Williamson

Geoffrey Arthur Williamson was an English classicist.He was a graduate of Oxford University and was Senior Classics Master at Norwich School from 1922-1960....

's translation of Eusebius of Caesarea

Eusebius of Caesarea

Eusebius of Caesarea also called Eusebius Pamphili, was a Roman historian, exegete and Christian polemicist. He became the Bishop of Caesarea in Palestine about the year 314. Together with Pamphilus, he was a scholar of the Biblical canon...

's Ecclesiastical History

Church History (Eusebius)

The Church History of Eusebius, the bishop of Caesarea was a 4th-century pioneer work giving a chronological account of the development of Early Christianity from the 1st century to the 4th century. It was written in Koine Greek, and survives also in Latin, Syriac and Armenian manuscripts...

gives the dates of Abgar's reign as from 179-214.

During the reign of Abgar the Great, Christians were favored in the realm of Osroene. It is thought by some that this led to the story of the letters between Abgar V and Jesus of Nazareth. Adolf von Harnack and many later scholars believe a corruption of a story of Abgar IX corresponding with Pope Eleuterus

Pope Eleuterus

Pope Saint Eleuterus, or Eleutherius, was Bishop of Rome from about 174 to 189 . He was born in Nicopolis in Epirus. His name is Greek for free....

 was the origin of the legend of King Lucius of Britain

Lucius of Britain

Saint Lucius is a legendary 2nd-century King of the Britons traditionally credited with introducing Christianity into Britain. Lucius is first mentioned in a 6th-century version of the Liber Pontificalis, which says that he sent a letter to Pope Eleuterus asking to be made a Christian...

, who supposedly introduced Christianity to his realm.