After the death of his mother Cleopatra Thea ca. 118 BCE, Antiochus IX Cyzicenus challenged his half-brother, Antiochus VIII Grypus, for power of Syria.
He returned to Syria in 116 BC to claim the Seleucid throne from half-brother/cousin Antiochus VIII Grypus, with whom he eventually divided Syria, with the right to rule half of the Seleucid Kingdom granted to him in 116 BCE.
Antiochus IX Cyzicenus was first married to Cleopatra IV, who was then killed in 112 BC by her sister and rival Syrian Queen Tryphaena, wife of King Grypus. After the death of Grypus and capturing the capital, Antiochus IX Cyzicenus married Cleopatra Selene I, the sister of his former wife Cleopatra IV.
Beginning his reign in 95 BC, the son of Antiochus IX 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.