Watchdog Man

In a world of fantasy of magic and knights, a man from earth became a ruler of a small territory, and he secretly defeated the strongest demon king with his power to become invincible whenever he morphs into a dog. However, after taking the life of the strongest demon king, a system appeared before him, connected to the anime, movies, Marvel, DC, etc. multiverse that he knew.

He needed to do specific tasks or missions on that multiverse. If he failed or did not make a good result on those multiverse, any random apocalypse, aliens, demon gods, etc., would terrorise humanity again.

1st World: One Punch Man as Watchdog man.
2nd World: DC as Alfred.
3rd World: Pokemon, the Young Giovanni.
4th World: Marvel as the Beast, worshipped by the hand, a villain organization.
5th world: Ghost fighter as Raizen.
6th world: Hunter x Hunter as Maha zoldyck.