What is "khwaday-namag"

Khadāy-Nāmag ("Book of Kings") was a Middle Persian history text from the Sasanid era, now lost, imagined first by Theodor Nöldeke to be the common ancestor of all later Arabic-language histories of the Sasanian Empire. It was supposed to have been first translated into Arabic by Abd-Allāh Ibn al-Muqaffaʿ (d. 757 AD), who had access to Sasanid court documents. According to Nöldeke's theory, the book itself was composed first under the reign of Khusraw Anushirvan, and redacted in the reign of the last Sasanid monarch, Yazdegerd III (r. 632–651). The Arabic translation of Khwaddy-namag was the primary source of Persian book Shahnameh (Book of kings) written by Abul Qasim Firdausi.

This view of Nöldeke's has recently been disproven.