Last Boss no Mukogawa - Saikyo no Ura Boss = Jashin ni Tensei shitakedo, 1000-nen Dare mo Konai kara Gakuen ni Kayou Koto ni shita (Novel)

The short life of a 16-year old boy, who had a love of observation, ended after an accident, after which God appeared. The boy was a perfect candidate for a certain reincarnation, so he was asked if he wouldn't mind doing so. The boy gave his consent, then reincarnated into another world. He reincarnated into the Evil God Ashtal. Just like in a game, once someone defeated the Last Boss, the Demon King, they would then be met with a bonus adversary, the overwhelmingly powerful Hidden Boss. He spent 1,000 years observing the world through his magic. Hey! Why hasn't anyone made it here yet?! I'm supposed to be the Hidden Boss! Finally, Ashtal became bored out of his mind; and after a certain incident, he decided to attend school. Sometimes he has to beat down a demon that gets carried away with itself, and sometimes he has to become a bank president to counterattack a great nation that got carried away with itself. This is that kind of Evil God story.