Everyone Else is a Returnee

Left out during the elementary school picnic. Left out during the middle school camp. Left out during the high school trip. I finally became a college student, and what? I’m left out from all of humanity? Ignored by God, Yu IlHan spends a millennium sharpening his skills for the Great Cataclysm, watching over Earth alone while everybody else is away in other worlds. His legend starts after humanity comes back and meets the Great Cataclysm!