Ayanashi

Humans now live underground, avoiding monstrous creatures that dominate the surface and are resistant to all known weapons. The only beings capable of defeating these creatures are the Ayanashi. In an underground city, two siblings meet a badly injured boy named Holo, who gradually becomes close to them. A monster soon attacks the city, destroying everything in its path. Holo confronts it, wielding a flaming short sword attached to his arm. This marks the beginning of a journey to restore light to a world trapped in darkness.