In 334 BC, Alexander the Great invaded the Achaemenid Empire, defeating the last Achaemenid emperor, Darius III, at the Battle of Issus. Following the premature death of Alexander, Iran came under the control of the Hellenistic Seleucid Empire. In the middle of the 2nd century BC, the Parthian Empire rose to become the main power in Iran, and the century-long geopolitical arch-rivalry between Romans and Parthians began, culminating in the Roman–Parthian Wars. The Parthian Empire continued as a feudal monarchy for nearly five centuries, until 224 CE, when it was succeeded by the Sassanid Empire. Together with their neighboring arch-rival, the Roman-Byzantines, they made up the world's two most dominant powers at the time, for over four centuries.
Is there an answer to this question (If it cannot be answered, say "unanswerable"): Who was the last Archaemenid Emperor at the time of its defeat by Alexander the Great?
Darius III