According to the provided text from Chapter 18, what is the role of the Guru in a devotee's life?

Based on the introductory verses of Chapter 18, the Guru, or Sadguru, plays a supreme role in a devotee's life. The Guru is hailed as the 'supreme Eternal one' and the 'Ruler of Maya,' who shows the world to be an illusion through direct experience. The chapter states that the Guru can effortlessly point out the 'Unperceivable One' that even the Vedas and Shrutis debate endlessly. Furthermore, the text explains that when a devotee receives the Guru's grace, the false logic of 'mine' and 'thine' disappears, leading to a state beyond duality and establishing the Guru as the teacher of Self-knowledge.