Last Boss no Mukougawa

A 16-year-old boy who enjoyed observing the world dies in an accident and is chosen by God for a unique reincarnation. He is reborn as Ashtal, an evil god destined to act as a hidden boss in an RPG-like world, appearing only after the final boss is defeated. Ashtal spends over a thousand years waiting for heroes to challenge him, but none arrive. After 1005 years of solitude, a group of female heroes led by Yufilia Plantagenet, the second princess of the Britton Kingdom, confronts him. Following an awkward encounter, Ashtal decides to abandon his role as an observer and instead attend school with the heroes who came to defeat him.