Going Back in Time: Counterattack After the Failure of Ascension to Immortality

What should I, a 1,000-year-old monster, do when I fail to ascend? Traveling through time and space and accidentally returning to my 9-year-old body, of course I need to practice again. But what happens after that? Revenge! Wow hahaha But the idiot who killed me was accidentally the protagonist of the stupid x harem fairy tale drama I was reincarnated into. But wait, isn't he only seven or eight years old at this time? Well, since I can't hurt a child or the main character without getting hurt in some way, I can only raise him to be better, faster, stronger, and more loyal! This novel and its characters are purely fictitious. Any similarity is purely coincidental and should not be imitated.