What is "chihrdad"

Čihrdād nask is one of the lost nasks of the Avesta and survives only as a summary preserved in Dēnkard 8.13.

In the summary, the text is said to have been a history of mankind from the beginning down to the revelation of Zoroaster, and it was an important source for later works like the Šāhnāmeh of Ferdowsi.