Based on Chapter 10, what is the role of a Sadguru in a devotee's life, and how can they alter one's destiny?

According to the teachings in Chapter 10, the Sadguru is paramount for spiritual liberation and can fundamentally alter a devotee's life. The chapter states that by placing faith in the Guru's feet, He "strikes a nail over the lines of destiny," effectively changing one's fate. The Sadguru is described as a boatman who helps a devotee cross the "ocean of existence" (Bhavasagar) and overcome the three types of miseries. Through pure service to the Sadguru, the cycle of worldly existence is destroyed, and liberation, characterized by bliss, is given effortlessly.