Strictly speaking, Yongyi Duke is not the one leading the army; he has already left the martial circle.
The Imperial Court has spies in the Rong army, and they know the Rong have retreated, and Mount Zhige almost wiped them out.
At this time, the Imperial Court should seize the opportunity to recover lost territory, but a problem arises: how will the Immortals of Mount Zhige view the Imperial Court's army? When approaching the mines, will we suffer the same fate as the Rong's army?
Of course, the Imperial Court has no intention of reclaiming the mines, as engaging an Immortal comes with a cost.
However, there are some with ulterior motives who believe the Imperial Court should take over the mines—after all, they're just useless iron mines, the Immortals probably wouldn't mind, right?
The Imperial Court's response to these people is: Fine, go ahead and inform them.
These are trivial side issues, the real question is: How do we communicate with the Immortal occupying the mines?
