Shinwa Densetsu no Eiyuu no Isekaitan

Hiro Oguro is summoned to Aletia, a world he once ruled as the Hero King Mars. He had previously established the Grantzian Empire before erasing his memories and returning to Japan. A thousand years later, the empire is in decline under a warlike ruler, with rival nobles vying for control and external threats looming. Meeting Princess Elizabeth von Grantz pulls him into the conflict over the imperial throne. He must once again take on the role of the legendary warrior to protect Aletia.