I've Become a Great Demon Cultivator, and Darkness Is Only Just Descending?

My name is Xu Yang. I once dreamed of becoming a martial arts prodigy. Then everything fell apart. That wretched Longevity Medicine and its disguised Martial Arts Prodigy Assistance System sent me tumbling into the demonic path! Longevity Medicine: “Xu Yang! How can you blame an injured, amnesiac medicine of justice like me?” “A medicine of justice from an evil cult?” “Slander!” “That is the worst slander imaginable against a medicine of justice!” “Wait! Xu Yang! Why am I mentioned in every single volume of your Book of Repentance?!” “Should I have written more?” “You got it wrong! For example, what's wrong with the first rule, joining a powerful faction?” “A powerful faction devoted to restoring the dark world? You think that's right?” “Then what about rule one hundred and eight, to...” Many years later, darkness descended. Xu Yang ascended the platform of eternal night and heard the title by which the dark world proclaimed him. Spirit Lord, Ruler of Returning Embers.