You were sent undercover, not to become the Demon King

Chen Mu is framed by Zhou Yuan, a disciple of the righteous Yunyin Mountain, and forced to go undercover in the Blood Demon Sect. During the entrance test, his system activates; by visualizing scenes from disaster movies, he generates an SSS-rank "Evil Value," causing celestial phenomena. Impressed, the Sect Master, Xue Wuya, takes him as a direct disciple and gifts him the sect's supreme treasure, the Blood Sutra. He achieves Foundation Establishment after just one night of cultivation, shocking the entire sect. Later, he practices dual cultivation with his senior sister, Liu Ruyan, causing his power to skyrocket to the mid-Golden Core stage, and he is appointed as the Saint Son. Meanwhile, Zhou Yuan uses Chen Mu’s sister as leverage, forcing him to steal the Blood Sutra. Chen Mu alters the scripture to include the prerequisite "To master this technique, one must first castrate oneself" before handing it over. Consequently, Zhou Yuan and his cohorts castrate themselves to practice, becoming laughingstocks. At the Righteous-Demonic Grand Competition, Chen Mu appears as the Saint Son of the Demonic Sect, shocking the righteous cultivators. Zhou Yuan and others attempt to kill him to gain fame, but are easily crushed by Chen Mu. The story revolves around the dual themes of his "undercover identity" and his "rise as the Demon Lord." Ultimately, Chen Mu will lead the Demonic Sect to counterattack Yunyin Mountain, rescue his sister, and execute his enemies.

Synopsis from DramaGo.