In Khorasan in northeastern Iran, a 10th-century Iranian nobleman brought together four Zoroastrian priests to transcribe a Sassanid-era Middle Persian work titled "Book of the Lord" ("Khwaday Namag") from Pahlavi script into Arabic script. This transcription, which remained in Middle Persian prose (an Arabic version, by al-Muqaffa, also exists), was completed in 957 and subsequently became the basis for Firdausi's "Book of Kings". It became enormously popular among both Zoroastrians and Muslims, and also served to propagate the Sassanid justification for overthrowing the Arsacids (i.e., that the Sassanids had restored the faith to its "orthodox" form after the Hellenistic Arsacids had allowed Zoroastrianism to become corrupt).