“Now King Alexander assembled large forces and encamped opposite King Demetrius. The two kings met in battle. The army of King Demetrius fled. King Alexander pursued him and defeated them. He pressed the battle strongly until the sun set. On that day King Demetrius fell.”
King Alexander from Ptolemais had a large army against King Demetrius I of Antioch, as the 2 kings met in battle. The army of King Demetrius I fled. King Alexander pursued, defeated, and killed him. So now King Alexander I, the supposed brother of King Antiochus V, was in charge of the Seleucid dynasty.