Tsuihou sareta Ochikobore, Henkyou de Ikinuite S-Rank Taimashi ni Nariagaru

This world is ruled by the Twilight. 150 years ago, war broke out between humans and demons. The humans suffered massive casualties as a mysterious phenomenon called "Twilight" took over the world. In that world, there exists a certain average boy. One day, unfairly abandoned by his party members, he's consumed by the Twilight.

Two years pass. In the uninhabitable twilight, that boy struggles like mad to survive. Eventually, he returns to the barrier city with the powerful skills he acquired in the Twilight. What awaits him next? No matter what, he moves forward, believing that a blue sky awaits him beyond.

(Source: Shousetsuka ni Narou, translated)