The Secret Spirit Area chosen by Chen Guan is called the Earth Evil Abyss. The environment there is very special; it's not suitable for large-scale battles, but it contains an abundance of high-level Secret Spirits, almost all at the Sixth Sense level.
It can be said that it is a forbidden zone for humans, and even top-level Sixth Awakening Powerhouses wouldn't venture there lightly.
Of course, the most crucial point is that this Earth Evil Abyss is not within Chen Guan's sphere of influence.
When using resources, of course he would use others' resources. Isn't it more favorable to keep the family's resources for his own followers to use?
The Earth Evil Abyss lies within the Shang Family's territory, making it hard to say this wasn't deliberate on Chen Guan's part.
Although the Shang Family later did not participate in the Gong Family's actions against Purple Sun City, Chen Guan still remembered their previous conduct.
