At the age of twenty-five he happened to hear the homilies of Hystaspes, the Bishop of Edessa, received instruction, was baptized, and even admitted to the dioconate or the priesthood.
#according to this idea Grotefend was able to identify X for Xerxes, Y for Darius and Z with Hystaspes.
Vishtaspa (fl. 10th century BCE or earlier, if actually historical), the first patron of Zoroaster
The fourth century Ammianus Marcellinus (xxvi.6.32) identifies Zoroaster's patron with another Vishtaspa, better known as Hystaspes in English, the (late-6th century BCE) father of Darius I.