Fランク?いえ、SSS+です

Ten thousand years after the nuclear war, recluse Silas dwells in the wasteland’s Grey Zone. He casually shatters an S-class radiation storm with a slipper, spotted and recruited by his former apprentice, Imperial General Valeris. Dismissed by aides as a powerless F-rank weakling, he casually slays an S-class beast, yet is only deemed lucky. He later reveals his forbidden SSS+ strength, eliminates traitors and foes, and sacrifices himself to slay the apocalyptic Glutton.