The fights don't just happen between the two sides, but the groups themselves fight each other to get the precious resources that the Devourer Demon promised.
The people who killed so much were the twelve cultivators of the Body Transformation Stage. They even ignored everything about the alliance and killed their allies in search of benefits. But other alliances were also formed to share the rewards.
Two small voids were opened and four items were thrown at them.
In exchange for four items very precious to cultivators in the Body Transformation Stage, the Devourer Demon wanted the death of four cultivators of that same stage.
The four items offered in exchange were pills, but special pills that can clean all the accumulated impurities from the cultivator's body, make all his energies purer, and still give the longevity of two hundred and fifty years.