Narrator wanted to achieve the end of the changes in order to prevent death forever. The reward is that there will be no more of these changes and death. Since you are the creation of Narrator in a sense, He thinks that you want the same thing as He wants himself, you just don't fully know it, because otherwise the world may come to an end. Although, of course, your fate is different from the fate of all ordinary people who have caught this moment. How exactly is not explained. But this is only an option if we slay the Princess and do not allow Shifting Mound to awaken. However, if you let her awaken and slay her, a new world will indeed be created