The first generation of disciples in the Demon Sect were all picked up by Yan Li during his travels around the world. He unintentionally collected too many, so he straightforwardly established a sect of his own.
Indeed, the establishment of the Demon Sect was really done so casually.
Back then, Yan Li wasn't the world's leading Sword Cultivator and the twelve original disciples of the Demon Sect were merely invisible figures within the cultivation world. But as time progressed, they were constantly advancing at an astonishing pace, thanks to their respective opportunities and adventures.
Of course, more than half of the credit for this goes to their mentor.
Gradually, the Demon Sect became stronger and grew to the size it is now.
However, the path of cultivation is a road leading to the heavens, with countless falling on the way. Even the initial group of disciples of the Demon Sect couldn't avoid this fate.