S-Rank Horrors Are Fighting to Be My Sidekicks

In the year 3026 of the Blue Star calendar, unknown creatures swept across the globe. They were indescribable and impossible to look at, and no one knew whether anyone would survive inside the fog. Every fog-shrouded area has a lord. Defeat him, and your supernatural abilities will level up. Rules for Blue Star residents: 1. Don't go near the fog unless you have a death wish. 2. The monsters are cunning and treacherous. Don't trust them. 3. If you accidentally come into contact with the fog, find a safe place and wait for rescue. The protagonist is accidentally caught in an S-rank fog, fighting monsters and leveling up all the way. Later, they discover that most of these powerful S-rank monsters are underlings they beat up in their dreams. (I haven't decided on the protagonist's name yet, so I'll just call them the protagonist. The blurb may be revised, but the general plot won't change.) Theme: Work hard to become stronger!