According to Chapter 18, what is the nature of the Sadguru and what happens when a devotee approaches them?

Chapter 18 describes the Sadguru as the supreme, eternal one, the truth of Brahman, and the ruler of Maya who teaches the knowledge of the Self. The chapter presents a powerful analogy to explain what happens when a devotee approaches the Guru: just as a salt doll that enters the ocean to bathe can never return, so too is the case for one who approaches the Sadguru. This implies a complete dissolution of the individual ego into the vastness of the Guru's grace and consciousness, a concept beautifully illustrated in the opening verses of this chapter.