Ancient Iranian religion was strongly connected. Beginning in the early 10th-century BC, the ancient Iranian religion was gradually displaced by Zoroastrianism, which contains many essential aspects of its predecessor. The sources on ancient Iranian religion, though limited, consist of textual and material sources. The textual sources are both Iranian and non-Iranian. An important Iranian source is the Avesta, which are Zoroastrian sacred scriptures made in the Avestan. This is considered the principal source of knowledge on ancient Iranian religions. It is a collection of several texts that seems to have been over a large span of time by a variety of authors. These
Avesta in the royal archives ("Dk" 4B, 5). Following Alexander's conquest, the Avesta was then supposedly destroyed or dispersed by the Greeks after they translated the scientific passages that they could make use of ("AVN" 7–9, "Dk" 3B, 8). Several centuries later, one of the Parthian emperors named Valaksh (one of the Vologases) supposedly then had the fragments collected, not only of those that had previously been written down, but also of those that had only been orally transmitted ("Dk" 4C). The "Denkard" also transmits another legend related to the transmission of the Avesta. In that story, credit for collation and