Avesta, also called Zend-avesta, sacred book of Zoroastrianism containing its cosmogony, law, and liturgy, the teachings of the prophet Zoroaster (Zarathushtra). The extant Avesta is all that remains of a much larger body of scripture, apparently Zoroaster’s transformation of a very ancient tradition. The voluminous manuscripts of the original are said to have been destroyed when Alexander the Great conquered Persia. The present Avesta was assembled from remnants and standardized under the Sāsānian kings (3rd–7th century AD).
The Zend-avesta, James Heyworth Mills