In the past there has been a
great deal of speculation about who the Ashen Hide were worshipping. I think that's over. Per the Ssethregore Codex the question of who they worship is now clearly Nier, even though they still refer to Him as the Fire Dragon:
On page 136 of 5e Ssethregore Henry wrote:
Eventually, a compromise was reached. A [Cancerese] human priest of Nier would be assigned to lead the ss’ressen congregation in worship, teaching their priestesses only the most basic of cants, no more than would be taught to templars and holy champions. The ss’ressen took this news with equanimity, seeing it as a test of their faith and stoically accepted whatever limitations were placed upon them.
Over the decades, these edicts have not been as strictly enforced as they were in the beginning. The Ashen Hide have proven their loyalty and fervent adherence to the God of Flaming Destruction, butchering as many Black Talon warriors as humans during the endless conflicts between Canceri and Milandir. A Cancerese priest of Nier, currently Hurson val’Virdan, remains to instruct and guide the ss’ressen priestesses.
The cants being taught and used by the Ashen Hide are Dark Triumvirate Nierite cants. They are not worshipping Kassegore