"In bygone days, all the lands had submitted to the Shah. It came to pass that Eranshahr went to war with the kingdom of the Romans. At first there was complete and total victory, and then chaos. Both kingdoms ended up devastated. The beneficiary was the Arabs. Coming out of the southern lands like a tidal wave, they swept aside all before them. Their new heresy threatened to exterminate the flame of the Zoroastrian faith. They very nearly succeeded too: but for the intercession of a great warrior. Now known to be a fraudulent claimant in spite of his merits, it was "Ardashir III," resurrected from his untimely death that proved the savior of Persia. It was Year 23 of the Yazdegerdi era, and 1223 years since the birth of the prophet Zoroaster. Ardashir was likely a minor general in the Sassanid armies, who was able to achieve prominence thanks to loyal soldiers."
-The Persian Chronicle
1)- Eranshahr was the self-given name for the Sassanid Empire.
2)- Ardashir III reigned very briefly, being killed in a coup at the age of nine in 630.
3)- the year attested is 655 AD. Calculations for Zoroaster's birth at the time were inaccurate, and this is reflected. Additionally, time was usually told by the regnal era. For the purposes of this TL, a new calendar will be introduced starting from this year, commemorating the reign of the usurper Ardashir.
this is just a preview really... more is to come.