Gu insects are best known from such a mystical place as Miao Jiang, which has a very long history, especially since the Yunteng family has taken to the path of the Evil Gu, which is a heretical practice.
Placing a bunch of poisonous creatures in one spot, in the end, only one remains.
Teng Huayang had learned the Gu Art from a young age, and he also carried Poison Gu with him at all times. Unlike the common people of the Yunteng family, he held the power over many people's life and death.
Take, for example, a master like Teng Dong, who had Gu insects implanted in him since childhood—if he harbored any disloyalty towards the family, the head of the family could easily manipulate his life or death.
His trip to Jiangcheng this time, albeit primarily for Ning Fan, also required Teng Huayang to check in on Shangguan Tenglong and others, due to Teng Zhuhai's reasons.