Shinwa Densetsu no Eiyuu no Isekaitan

Once upon a time, a boy was referred to as a "War Hero." In another world called Aletia, the boy who had saved the Lost Kingdom, conquered their neighbouring countries, and led to the rise of a grand empire decided to throw everything away, returning to the modern era at the cost of his memories. As three years passed, the boy who was happily enjoying his daily life was once again called back to the other world. However, what awaited him was the Aletia of 1000 years in the future. As his past glories became a "myth," the legend of the youth referred to as the, "Twin Black Hero King," begins.