Creatures in the game are categorized in some "general" groups. These general categories are indeed more general than creatures' "races". Hence, creatures with different race may (or may not) fall in the same "general" category.
The general categories enemies may fall in to are these:
- humanoid
- animal
- undead
- gianthumanoid
- monster
The term "unnatural" in the club's description refers to the "monsters" (i.e. only those creatures whose "general" category is "monster).
Examples: Undead Creatures and golems don't fall in this category. Otyughs, spiders and Trolls do, but Giant Trolls don't!
If you have any particular creature in mind you can either check it in an editor to see its "general" category or you can just ask me.