Jiu Yin was not one of the Darkness Tribe.
He was originally a vicious beast that contained all the Yin qi of the realm.
However, he got into conflict with the Radiant Church and killed many of the church's cultivators, hence getting labeled as one of the Eight Great Heretics.
The reason why he disliked Hei Xuan was that they were both powerful and ferocious beasts, who simply fought because of their natural instinct.
If it were just the competition between two vicious beasts, it would not be anything worthy of people's concerns.
However, Hei Xuan detested Jiu Yin's method.
Jiu Yin cultivated the Yin Qi Dao, and the Nine Yin Heavenly Art that he cultivated was a strange technique. It allowed him to absorb the Yin core of other cultivators to strengthen himself, especially the Yin cores of women.
