The text across hundreds of books, modules, guides, maps, and other materials that describe various classes, items, spells, and other elements is 100% copyright

The text across hundreds of books, modules, guides, maps, and other materials that describe various classes, items, spells, and other elements is 100% copyrightable, and anything that is "substantially similar" would be infringing on those copyrights. So yes, you're right, you can't copyright the real world rule for a game, but with enough existing copyrighted text, good luck describing similar rules in a similar universe with similar naming in a way that isn't substantially similar.