Cocytus - Reference

White Dragon Bone made the wearer only as strong as the amount of intelligence they forfeited. On the body of someone wise, it could be harder than not only steel but also mythril or the legendary adamantite.

The catch was that any cleverness stolen by the armor would not return when the wearer took it off—thus its reputation of being cursed.

When the Razor Tail chief, famous for his wisdom, equipped the armor, he was able to repel all the weapons the lizardmen had. Its fellow Great Treasure Frost Pain was no exception. The armor had probably reached adamantite-level hardness.

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Volume 4, Chapter 3.1