I feel like the start skipped a couple chapters worth of information even though I know it didn't. It's as if the story started on chapter 5 or even later. The information that is missing at the start of the story, is gradually revealed over the first few volumes (some info still hasn't been revealed yet though) This manga is about a 200 year old strongest magician who reincarnated into a commoner on another continent (and no one is aware of the existence of other continents except for the MC, since no one can cross the sea). The story is essentially about the MC trying tofix a corrupted empire and fighting against corrupted nobles.
The MC is overpowered but also not invincible.