Life-or-Death Challenges

One day, a Taoist priest receives a mysterious letter at home. When he reaches the specified location, he falls asleep without realizing it. Upon waking, he finds himself surrounded by many people. Stranger still, they are all companions who once fought alongside him. What is going on? They have accidentally entered a series of challenges, where a mysterious person is controlling them. Who is this mysterious person, and how will they fare as they face the challenges?