Time Travel: Kidnapping the Protagonist and Becoming His Little Brother

What is it like to travel through time to a novel about a harem that cultivates immortality? Even though I escaped the fate of being taken into the harem by the protagonist, after thousands of years of hard training, I was killed by the idiot protagonist when I was ascending to enlightenment. After my death, I was transported back to the beginning of the novel. This time I must find the protagonist and make him pay with his blood!