Xerxes.

He succeeded his father Arsames I

Arsames I

Arsames I seems to have taken control of Commagene, Sophene and Armenia in the year 260 BC after the death of his grandfather Orontes III, king of Armenia, and his father Sames, king of Commagene....

 to rule both Sophene

Sophene

Sophene , or ) was a province of the Armenian Kingdom and of the Roman Empire, located in the south-west of the kingdom. It currently lies in modern-day southeastern Turkey....

 and Commagene in 228 BC, his brother Orontes IV ruled Armenia

Armenia

Armenia , officially the Republic of Armenia , is a landlocked mountainous country in the Caucasus region of Eurasia...

.
The Satraps of Upper Media, Lower Media

Molon

Molon or Molo was a general and satrap of the Seleucid king Antiochus the Great . He held the satrapy of Media at the accession of that monarch ; in addition to which, Antiochus conferred upon him and his brother Alexander the government of all the upper provinces of his empire...

 and Persis Persis

Alexander (satrap)

Alexander was brother of Molon. On the accession of the Seleucid king Antiochus III, afterwards called the Great, in 223 BC, he entrusted Alexander with the government of the satrapy of Persis and Molon received Media...

 rebelled against king Antiochus III

Antiochus III the Great

Antiochus III the Great Seleucid Greek king who became the 6th ruler of the Seleucid Empire as a youth of about eighteen in 223 BC. Antiochus was an ambitious ruler who ruled over Greater Syria and western Asia towards the end of the 3rd century BC...

 in 223 BC.
Also in 223 BC Achaeus

Achaeus (general)

Achaeus was a general and later a separatist ruler of part of the Greek Seleucid kingdom. He was the son of Andromachus, whose sister Laodice II, married Seleucus Callinicus, the father of Antiochus III the Great. Achaeus himself married Laodice of Pontus, one of the daughters to Laodice and...

, the Satrap of Asia Minor

Asia Minor

Asia Minor is a geographical location at the westernmost protrusion of Asia, also called Anatolia, and corresponds to the western two thirds of the Asian part of Turkey...

, rebelled.
By 220 BC he put down most of the rebellions, Achaeus was not defeated until 213 BC.

All of these rebellions helps to understand the context, and motives for the subsequent actions of Antiochus III on king Xerxes.
By 212 BC Antiochus III had invaded the domain of Xerxes and defeated him after laying siege to the city of Arsamosata

Arsamosata

Arsamosata was a city in Armenian Sophene near the Euphrates. It was founded by King Arsames I of the Orontid Dynasty in 3rd century BC. It was left and destroyed in I century BC. In Middle Ages it was called Ashmushat....

. Shortly afterwards Antiochus III arranged for Xerxes to marry his sister, Antiochis

Antiochis

The name Antiochis, in Greek Ἀντιoχίς is the female name of Antiochus. Antiochis in Greek antiquity may refer to:-Hellenistic queens consort:*Antiochis, daughter of Achaeus, married to Attalus, and the mother of Attalus I, king of Pergamon...

.
This move seems to show the power or influence Xerxes had in the region. However within the same year she arranged to have her new husband assassinated, thinking that Antiochus III would be able to take control of Sophene. Whether Xerxes had still ruled Commagene by the time of his assassination is not known.

Antiochus III had many military campaigns to take command of and so rather than ruling Sophene he seems to have preffered to place Sophene under the rule of Xerxes son, Abdissares (Zareh).