Yan Sima

Sima Yan was a gloomy-faced and ambitious cultivator who served as the vice commander of an army of ten thousand demon soldiers.

[Spoiler] Operating under Commander Wang Lin, he played a key role in the military affairs surrounding Wang Lin's rise to command. Known for his calculating demeanor and deep killing intent, Sima Yan's eventual downfall came as a result of his cruelty, ambition, and defiance against Wang Lin's authority. [/Spoiler]

(Source: Xian Ni Wiki)