To put it briefly, you are a Knight. Sworn by oath to your fair lady. You committed acts of good. You committed acts of evil. All of it in her name. And for the sins you committed as her knight, you've been condemned to the darkest pit of hell. But your oath does not waver.
Cleave your way through hell, with the idea of multiple worlds created for the specific sins all of humanity has made that you must travel through. Death is only starting from the beginning again. Cut through Demons, Sinners, The Devil, and everything in between whilst you harness the powers that be for the sake of returning to your fair lady.
I don't want to go much further in detail. But that's just the most basic of gists I'm comfortable with giving.