Plutarch, Themistocles
Bernadotte Perrin, Ed.

At break of day, Xerxes was seated on a high place and overlooking the disposition of his armament. This place was, according to Phanodemus, above the Heracleium, where only a narrow passage separates the island from Attica; but according to Acestodorus, it was in the border-land of Megara, above the so-called ‘Horns.’ Here a gilded throne had been set for him at his command, and many secretaries stationed near at hand, whose task it was to make due record of all that was done in the battle. [2]

But Themistocles was sacrificing alongside the admiral's trireme. There three prisoners of war were brought to him, of visage most beautiful to behold, conspicuously adorned with raiment and with gold. They were said to be the sons of Sandauce, the King's sister, and Artayctus. When Euphrantides the seer caught sight of them, since at one and that same moment a great and glaring flame shot up from the sacrificial victim and a sneeze gave forth its good omen on the right, he clasped Themistocles by the hand and bade him consecrate the youths, and sacrifice them all to Dionysus Carnivorous, with prayers of supplication; for on this wise would the Hellenes have a saving victory. [3] Themistocles was terrified, feeling that the word of the seer was monstrous and shocking; but the multitude, who, as is wont to be the case in great struggles and severe crises, looked for safety rather from unreasonable than from reasonable measures, invoked the god with one voice, dragged the prisoners to the altar, and compelled the fulfillment of the sacrifice, as the seer commanded. At any rate, this is what Phanias the Lesbian says, and he was a philosopher, and well acquainted with historical literature.