After the death of his mother c. 118 BC, Antiochus IX Cyzicenus challenged his half-brother, Antiochus VIII Grypus, for power over Syria.
He returned to Syria in 116 BC to claim the Seleucid throne from his half-brother/cousin Antiochus VIII Grypus, with whom he eventually divided Syria, that same year.
Antiochus IX Cyzicenus was first married to Cleopatra IV, who was then killed in 112 BC by her sister and rival Tryphaena, wife of King Grypus. After the death of Grypus and Antiochus' capture of the capital, Antiochus married Cleopatra Selene of Syria, the sister of his former wife, Cleopatra IV.
Antiochus IX probably created the Iturean tetrarchy as an ally against Antiochus VIII. Beginning his reign in 95 BC, Antiochus X Eusebes' first achievement was to defeat his double half-cousin/second cousin Seleucus VI Epiphanes, thus avenging the recent death of his father, Antiochus IX Cyzicenus. The epithets he took tell much of his story: Eusebes (being a title of his father) and also Philopator (father-loving) both honoured his father. After that, he ruled Antioch and its surroundings, fighting endlessly against the four brothers of Seleucus VI, the Nabataeans and the Parthian Empire.