True Demon Association was founded by a Demonic True Monarch cultivator a hundred years ago. Its goal is to overthrow the rule of the Nine Yang Sect and turn the Nine Yang Realm into the Demonic Regions.
But in Yu Xian's translation, it's basically an organization of independent demonic cultivators banding together for survival. They talk big, but most of the members are demonic cultivators from within the Profound Sun Region. It has not yet stepped out of the Profound Sun Region.
At the same time, the requirements to join the True Demon Association are basically non-existent, as long as one cultivates demonic skills, they can join.
To attract more demonic cultivators to join, there are no restrictions on the members of its peripheral organization, and they come and go freely.