Kokubyaku no Avesta by Masada Takashi. This is web novel version with some added illustrations from printed edition. I don't remember where did I get it, so credits for unknown anon. The formatting for the first books is honestly pretty bad, but I'm unfamiliar with Word and didn't managed to reformat it better. In time I probably will make another version with formatting as from original release (from enty), along with part of 事象地平戦線アーディティヤ what I have (up to 6th chapter), but for now it is better than nothing (t I didn't managed to find Japanese version of Avesta free anywhere, but maybe my google-fu is just that bad).