And now will I show you the truth. Behold, there shall stand up yet three kings in Persia; and the fourth shall be far richer than them all: and by his strength through his riches he shall stir up all against the realm of Greece.
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Aquinas Study Bible
Three kings: Cambyses, Smerdes Magus, and Darius, the son of Hystaspes. The fourth: Xerxes. (Challoner)
He states that four kings shall arise in Persia after Cyrus, namely Cambyses, |119 the son of Cyrus, and the Magus named Smerdis, who married Pantaptes, the daughter of Cambyses. Then, when he was slain by seven Magi and Darius had succeeeded to his throne, the same Pantaptes married Darius, and by him gave birth to Xerxes, who became a most powerful and wealthy king, and led an innumerable host against Greece and performed those deeds which are related by the Greek historians. For in the archonship of Callias he destroyed Athens by fire, and about that same time waged the war at Thermopylae and the naval battle at Salamis. It was in his time that Sophocles and Euripides became famous [hardly Euripides, whose first play was given in 455, nine years after Xerxes' death], and Themistocles fled in exile to Persia, where he died as a result of drinking the blood of a bull. And so that writer [apparently Tertullian, cf. p. 550] is in error who records as the fourth king that Darius who was ...