Histories

While Darius delayed making his decision, it chanced that at this time Demaratus son of Ariston had come up to Shush [48.333,32.2] (inhabited place), Khuzestan, Iran, AsiaSusa, in voluntary exile from Lacedaemonia after he had lost the kingship of Sparta [22.4417,37.0667] (Perseus) Sparta.

Learning of the contention between the sons of Darius, this man, as the story goes, came and advised Xerxes to add this to what he said: that he had been born when Darius was already king and ruler of Iran [53,32] (nation), AsiaPersia, but Artobazanes when Darius was yet a subject;

therefore it was neither reasonable nor just that anyone should have the royal privilege before him. At Sparta [22.4417,37.0667] (Perseus) Sparta too (advised Demaratus) it was customary that if sons were born before their father became king, and another son born later when the father was king, the succession to the kingship belongs to the later-born.

Xerxes followed Demaratus advice, and Darius judged his plea to be just and declared him king. But to my thinking Xerxes would have been made king even without this advice, for Atossa held complete sway.