pretty sure the demon king is a male kireina and his concubines are just evil versions of her wives
"Hoh, now that sounds interesting. Hah, doesn't it make it even more ironic that we're working together now?" Smiled the Demon King, caressing the head of one of her concubines, a gigantic lamia woman.
the legend this OP is talking about is an excerpt about champion monsters, it was stated that they are better then regular monsters and some have been seen to have multiple species following them