Being watched in the dark!

Just standing before the dense Death Qi was not an option as they could feel a threat from it, as such they retreated fast. 

From some distance, the corrosive nature of the grey mist made from fog accumulating seemed to have corrosive nature to any life.

Based on what he had learned, as long as death Qi got itself attached to someone, it would be impossible to get rid of it without extreme yang energy. But it is even more dangerous for those old individuals whose lifespan is on the countdown and once involved with it, it reduces one's lifespan. 

From the sect records, there is a Realm that has practitioners use the death Qi as a resource. They need to absorb it and convert it into life energy eventually, this is similar to the Nivarna stage. But it is called the life and death stage.

Even though the process is dangerous, it was made by someone, and for them to come up with such a system meant they had confidence it could work as it did.