That time the hero died and became a familiar

The hero gets out matched by the demon lord but his will to save those close to him drive him to develop an ultimate power to achieve anything by sacrificing his life, he does not hesitate to activate the power and kills the demon lord. After dying he finds himself in the palace of reincarnation where he learns the dark truth of the world, now he has to make a choice become a familiar and continue saving the world or reincarnate and live a new life. [participant in the Royal Road Writathon challenge]