Demon slayer: Jekyll and Hyde
Dr. Henry Jekyll, a great scientist and revered gentleman, had created a serum to seperate his evil ego from his good one, but ended up creating a seperate personality with it's own appearance, an entity called Edward Hyde, how cause various troubles and crimes, Henry even lost his friend Dr. Hastie Lanyon, died after the trauma of what he witnessed. After Henry's death in Edward Hyde form, he found himself in heaven judged by god that one he preach in church, Henry is willing to go to hell for atonement of past actions, but god who knew that henry meant no harm and alter ego Edward was just an accident, decided to send him in different earth to undergo a test he gave Henry, he didn't tell exactly what kind of test, as he simply told that it is upto Henry to figure out what test he is undergoing. Henry found himself in different earth in a cottage in middle of forest he age revert back to 15, though he can still feel Edward Hyde inside him, fortunately there is a note revealing the the impurity of salt that allowed the transformation and toggling happened in his past life is Sodium Iodide, allowing make the medicine again to keep his transformation under control. He checked the clothes he is wearing as well as his other clothes, they are all Japanese attires, Henry who had tour to various nation in his past life, immediately understood that he is in Japan, he decided to move out and socialize with Japanese people as he don't want become an odd one, just because he is British in Japanese clothes. He world however is not ordinary earth, but the earth of demon slayer world.