In anime, manga, or comics, it usually starts in the normal world. Then, something grants it power. Be it a virus, a world, a technology, or maybe even a profession. That thing would be what defines that world and sets it apart from the others.
It was cool to have powers. But they changed people. That was a common theme when they were introduced. The line defining a human usually blurred with the introduction of these foreign elements.
There were those that were acceptable and those that weren't. Usually, when the power was received in turn for a grave price, it changed the entire genre of the work. Because while everyone wished for more power, the price people could pay differed.
Funnily enough, if you were to classify the world of reapers, it would be dark fantasy. The ability to use souls was received at the price of death and eternal damnation.
"In exchange for a second life, Reapers forfeit the right to go to Heaven."