Where is the sentence you are referring to in the question?
Answer: The answer is action personification
Explanation: this is because the fog it’s not an actual living thing and a personification gives it human like personality and it’s doesn’t have a thought it has an action
The simple predicate is the one, main verb that is the action being performed. So here it would be the word “featured”.
Do you mean why didn’t Bilbo or Thorin kill Smaug?