The miasma in the deep parts of the rainforest was formed by being exposed to long periods of humidity and rot. If anyone breathed it in, they would feel weak and powerless. If they were exposed to it for a long time, then the Qi in their bodies would lose its liveliness. Gradually, they would become lethargic.
That was why the tribes around the area would usually only search for materials around the area and rarely venture in. Only the truly powerful Berserkers that constantly circulated their Qi when they entered could fend against the poison in the miasma and search within the deepest parts whenever they wanted.
At that moment, He Feng remained unmoving within the deeper parts of the rainforest that were thich with the formless miasma. His pupils shrank. His body was in an incredibly weakened state. He had no way of fighting back.
Li (里) is a unit of measurement in China. 1 li is equivalent to 500 m, or 0.31 miles.
I chose to use li instead of converting it to miles or using the metric system because:
1. If 1 li was equivalent to half a mile, I would have used miles, but as it were, saying "Please walk 310 miles into the distance, and you’ll find my tribe" sounded a little off to me, so I decided to retain the Chinese unit of measurement.
2. The metric system is the same. It is also too precise, and that sort of precision did not exist at that time.
Also if you want to know more about li, you can go to Wikipedia and type in li (unit). They go into great detail about the li system in a Wikipedia style there, and it’s pretty accurate.