Is the immortal octupole demon king good or bad for the protagonist?

Not good. The main purpose of octupole demon king to protect the protagonist in the fairy tale is to devour the soul of the protagonist and improve his cultivation. In the novel, the soul is an important foundation for practitioners to practice, and devouring the souls of other practitioners can quickly improve their cultivation, which is also the reason why Octupole chose Wang Lin as the target. The Octupole Demon King didn't realize that his behavior was actually challenging Wang Lin's bottom line, which also laid the groundwork for his failure. When the octupole demon king tried to devour Wang Lin's soul, Wang Lin chose to compromise instead of resisting. He agreed to octupole's request and gave his soul to him. However, just as the octupole prince was triumphant, Wang Lin suddenly launched an attack and sealed the soul of octupole prince in an inescapable seal with his Tao and cultivation. Generally speaking, the Octupole Devil plays an important role in Fairy Inverse. His existence promotes the development of the plot of the novel, and also reveals one of the themes of the novel: the conflict and transformation between good and evil. Wang Lin, on the other hand, successfully defeated the octupole demon king through both wisdom and courage, showing his strength and wisdom.