Achaeus (died 213 bc) A Seleucid general and rebel. In 223 he defeated Pergamon and won back the territory she had taken from the Syrian empire. In 220 he set himself up as independent from his king, Antiochus (3) III. He held out for a number of years until Antiochus penned him in Sardis and there besieged him for two years until he was finally betrayed and cruelly put to death.