The sacred scripture of Zoroastrianism, written in an otherwise-unrecorded ancient Iranian language called Avestan. Its oldest core, the Gathas, is a set of hymns traditionally attributed to the prophet Zarathustra and among the oldest religious poetry in any Indo-Iranian language. Carried orally by priests for many centuries and committed to writing only late, under the Sasanian Empire, the Avesta survives today as a fraction of a once-vast canon — most of it lost over the long decline of Zoroastrianism after the Arab conquest of Persia, the remainder preserved chiefly through Parsi and Iranian Zoroastrian manuscript lines.
Most of the Avesta is gone. The book it once was — a large library of scripture filling many volumes, recited from memory by generations of priests — was lost over the centuries of Zoroastrian decline that followed the Arab conquest of Persia, and what survives is perhaps a quarter of it or less. The pieces that remain are written in a language spoken by no one for over two thousand years, preserved in part by a small community that carried the manuscripts to India and guarded them for a thousand years. To read the Avesta is to read the ruins of a scripture, and to wonder what filled the missing shelves.