Twelve-Winged Dark Seraphim
I looked back at the mythology of a Fenghuang and saw it is supposed to be a bird with five "sacred colors". These being red, yellow, blue, white, and black. You can make any color by combining these five colors, so a Fenghuang being rainbow colored fits well. Another tidbit of mythology is that it's paired with a dragon as a symbol of harmony. Not sure if that means its status in mythology is similar to dragons, I just thought it was interesting. Of course, Fenghuangs might just end up being phoenixes in this novel, but we have no confirmations on that right now. So for now I'll just assume they're different.