It’s all well and good to know that your enemy has a weakness, but that information isn’t terribly useful unless you know what the weakness does to them.

It’s all well and good to know that your enemy has a weakness, but that information isn’t terribly useful unless you know what the weakness does to them.

The trope of the otherwise unstoppable creature with a single vulnerability is among the oldest in fiction or myth. Superman’s weakness to kryptonite is probably among the most famous, but well before that, Samson had hair, Achilles had a heel, Esfandiyār had his eyes, Baldr was susceptible to mistletoe, and half of the population of Russian folktales seemingly keep their hearts in ducks’ eggs or something else that can be conveniently found and crushed. Weaknesses make for good storytelling, not because it’s especially plausible for an antagonist to be nigh unstoppable but more because, once an antagonist seems to be unstoppable, characters get very excited at the importance of that word “nigh.” Smaug, Martians, Cybermen, and countless other examples pervade geekdom.

How do weaknesses work, though? Superman and Baldr are both vulnerable to specific substances, much like werewolves or vampires, whereas Achilles (or the similarly unprotected Talos) had small spots where they lacked their invulnerability. Broadly speaking, we might split all weaknesses into two categories: anatomic, those related to body structure, and physiologic, those related to how a body works. There’s obviously some overlap to this (was Baldr killed because he came in contact with mistletoe, or because he was grievously wounded with mistletoe?), and such a simplistic division doesn’t necessarily take into account, say, a creature whose only vulnerability involves disrupting protective spells or cutting their hair or something, but even so the ultimate effect of most kryptonites and heels falls into one of those categories.

Anatomic heels tend to be pretty straightforward. Sometimes they’re so straightforward as to be ridiculous; zombies, after all, are far from the only monsters that die if you shoot them in the head. The only real division between different types of anatomic weaknesses might be considered to be plausibility; it makes some sense to die if impaled on a wooden stake but less so to be killed by an injury to the ankle. Still, anatomic vulnerabilities often have an inherent logic to them that players can appreciate. Arguably, things like “I keep my heart outside of my body” qualify as anatomic vulnerabilities, even if they make no anatomic sense in and of themselves.

Physiologic vulnerabilities tend to be ones where the imagination can get a bit more exercise. Whenever we talk about physiology, we have a vast and varied array of ways that a body can stop working properly. Is a creature vulnerable to a particular substance because they’re allergic to it? If so, you might expect exposure to their weakness to be accompanied by choking, swelling, and all sorts of very impressive, rapid effects. Is a creature vulnerable to a substance because it’s poisonous to them? Superman loses his powers when exposed to kryptonite in much the same way that human cells lose the ability to transform oxygen into energy when exposed to cyanide, and he succumbs to “kryptonite poisoning” if the dose is too high. Any substance which disrupts some process within a body – a flow of energy, a chemical process, the transmission of electrical signals along nerves – is probably some sort of poison. This begs the next interesting question: what is a weakness’ mode of delivery? Most poisons need to be ingested or injected to affect humans, but oil-based substances can sometimes pass through our skin. Cybermen stop breathing if they inhale gold dust; presumably the particles are just the right size to clog a filter or something. Kryptonite affects Superman based on proximity – and in some comics, gave Lex Luthor cancer – because its harmful aspect is its radiation.

Weaknesses don’t have to make sense, but players generally like it when they can see some sort of connection between a creature and what it’s vulnerable to. Our desire to see such connections make sense explains why several hundred writers have postulated explanations for why silver hurts werewolves and why a few dozen horror novels trace vampires back to Judas Iscariot. Personally, I’d rather have no explanation for why a weakness works rather than a stupid one, but I’d rather have a decent explanation than none. A little bit of creative logic can give you that.

And then again, sometimes it’s perfectly fine to just say “it’s magic” and get on with things.

More than four years ago, Dr. Eris Lis, M.D., began writing a series of brilliant and informative posts on RPGs through the eyes of a medical professional, and this is the one that appeared here on May 24, 2015. Lis is a physician, gamer, and author of the Skirmisher Publishing LLC OGL sourcebook Insults & Injuries, which is also available for the Pathfinder RPG system.