Seijun Ecstasy: xx shinai to Derarenai Heya de Mazoku wo Buttaoshimasu

Ex is ridiculed by his peers as a failure of a hero, whose sacred light has yet to manifest. But it turns out to have manifested in his dick! How is he supposed to defeat anything with that?! Even when his town is attacked, all he can manage to do is have his crotch glow. While bearing the weight of these worries, a mysterious shrine appears before him...

(Source: Kodansha, translated)