Man dies. His last act in the previous life generates him an absurd amount of karma. He meets a god, and it reborns him in a crossworld of Harry Potter and DxD. He gets a gift, one that can only be fully explored with the knowledge that he learned in his previous profession in the previous world. The keeping of knowledge is also a gift. And with that, his karma is spent.
Thrown in the world with a 'good luck' and a slap in the back, he fights to survive until the start of canons.
The time until that, 1000 years.
Yeah… Now read about some of his adventures in this crossed over world, beginning already in HP canon.
English is not my main language, so you will find some strange stuff, like the mix of North American and the Queen’s English.
Disclaimer: All characters that you recognize from the franchise of Harry Potter and DxD are propriety of its respective creators and I only wish that they were mine. But they are not. I only own the MC, the OCs, and the ideas that generated the non canon plot.