The New Gate Vol. 9 (manga)

After defeating the final boss in THE NEW GATE, the online game's most powerful player, Shin, is transported to another world just like the in-game world. Shin is hard at work attempting to wipe out a massive swarm of monsters headed for the fortress city of Valmer. But then, an extraordinarily powerful monster appears. With his comrades in grave peril, Shin decides to activate some of his forbidden equipment.