Shiou no Kikan: Moto Yuusha no Ore, Jibun ga Soshiki Shita Chuuni Himitsu Kessha wo Tomeru Tame ni Futatabi Isekai ni Shoukan Sarete Shimau

Imagine being summoned to another world to be the hero, beat the Demon King, and live an adventure...only to be forced to return to modern-day Japan. That's what happened to Hizaki Shio. Those exploits seem like forever ago—he is in high school now—even though it's only been three years. But just as Hizaki has settled back into reality, he's suddenly summoned again and this time to stop the Secret Society, Helheim! However, Shio has a slightly cringey secret... He founded Helheim himself, during an embarrassing phase when he went around calling himself the Corpse King. It's physically painful to remember. So who could be using Helheim's name in his place, and how did it become so powerful? It's time to confront the marks he left behind. First stop: the Steel City of Gaghiel to rescue a noble heiress! (Source: J-Novel Club)