The system issued the mission so suddenly that it took Mag by surprise.
After returning from the Demon Islands, Mag had been studying the information about black fog and the Great Old Ones. Currently, he could only deduce that there was a batch of powerful and mysterious existences in this world in the ancient times. They could still exist till now in some other form.
The black fog should be a form of representation for some of these mysterious existences. They bewitched people who were not strong mentally, and made them become their servants willingly.
Furthermore, the black fog had been appearing more and more frequently recently, so this wasn't a good sign.
Mag wasn't sure if those mysterious existences had the power to break through the seals, or if they were already awakened from their slumber, and were trying to gain the control of this world again.