Can you tell me about the story that is introduced in Chapter 12 concerning a man from Nashik?

Certainly. Chapter 12 sets the stage for a story it calls "very delightful" and illustrative of the Guru's greatness. It introduces a man from the holy city of Nashik named Mule, who was a ritualistic and orthodox Agnihotri, meaning he was a keeper of the sacred fire. According to the chapter, this man came to Shirdi due to the strength of his past merits, as the text posits that one cannot stay in Shirdi even for a moment without such merit. It is also hinted that his original reason for being there was different from the eventual outcome, which led to him gaining the benefit of Shirdi and seeing his own Guru.