When Cid is isekai'd to another world, he creates an underground organization to live out a fight against a made-up cult. Little does he know the cult is real, and they're not happy his power fantasy just impeded their plans.

For as long as he can remember, Minoru Kagenou has been fixated on becoming as strong as possible, which has led him to undertake all kinds of rigorous training. This wish, however, does not stem from a desire to be recognized by others; rather, Minoru does everything he can to blend in with the crowd. So, while pretending to be a completely average student during the day, he arms himself with a crowbar and ruthlessly thrashes local biker gangs at night. Yet when Minoru finds himself in a truck accident, his ambitions seemingly come to a sudden end. In his final moments, he laments his powerlessness-no matter how much he trained, there was nothing he could do to overcome his human limitations. But instead of dying, Minoru reawakens as Cid, the second child of the noble Kagenou family, in another world-one where magic is commonplace. With the power he so desired finally within his grasp, he dons the moniker "Shadow" and establishes Shadow Garden: a group whose sole purpose is to combat the enigmatic Cult of Diablos, an organization born from Cid's imagination. However, as Shadow Garden grows in both membership and influence, it becomes increasingly apparent that the Cult of Diablos is not as fictional as Cid had intended.—MAL Rewrite

People aspire to be great heroes and evil villains, but Cid Kagenou is not one of them. He aspires to be the true brain behind it all, pulling the strings and directing the entire plot. He was unable to attain his objectives in his prior existence in modern-day Japan. But now, resurrected into a magical world, he will demonstrate the true majesty of darkness. As Cid poses as an ordinary mobster, he recruits members for his (false) group Shadow Garden, which aims to bring down the enigmatic cult that lurks in the shadows. Cid leads his (fictional) organization against the filthy (yet unexpectedly real) Diabolos Cult with plenty of ad-libs.—Bilibili TV