A nalurus looks like an ordinary person, but one who always wears a hood or mask. It may be pretending, even to itself, that it's still human, despite the terrible infection it survived but still carries. The nalurus's infection is transmitted by sight. If a living humanoid or related creature sees a nalurus without its hood and looks full upon the disquieting lines, spirals, and geometric shapes laid out in ridges across the creature's face, the awful pattern imprints on the victim's mind. Something in the interplay of information, refraction, and the physical structure of the victim's brain itself sets off a cruel and rapid chain reaction. What begins as a pinkish nose drip ends when the victim's brain completely liquefies and exits the victim's head from eyes, nose, mouth, and ears less than a minute after the infection occurred.
My concept for the nalurus was born from my real fear of a disease called Creutzfeldt–Jakob disease (CJD). CJD is an incurable and fatal brain disease that is caused by a type of protein called a prion. Prions are misfolded proteins that infect other nearby proteins merely by being close enough to incite those proteins to misfold. It becomes a sort of chain reaction that, over months, dissolves the brain. Obviously, over a billion years, it's gotten more aggressive . . .
But telling readers all about CJD in the Nalurus monster description itself, or the science behind some other monster I'm writing, despite it being my first instinct to do so, isn't actually the way to go to maintain the Numenera design esthetic. It turns out, I needed to deprogram, and I'm thankful the MCG team was there with the design and editorial insight that allowed me to do so. At the end of the day, Numenera is a game designed to inspire a sense of wonder, mystery, and weirdness in its players, not explain the science of how prions misfold, the underlying mechanism behind spurn evolution, or how an amber monolith might be able levitate for so many thousands or even millions of years. For most people, those specific explanations will come across as technobabble. Revealing the entire answer to a mystery is sort of like telling a joke, then before anyone can appreciate it, following up with "The reason that's funny is because [proceed to ruin joke here]."
We (I, Monte, and Shanna, if they'll allow me to speak for them) are not suggesting that a designer shouldn't have such background information in his or her back pocket, possibly even to reveal the edges of in a monster description if it seems right to do so, or indeed, available for some other purpose (for instance, such as a blog post like this one). We're just saying that the main body of a Bestiary monster entry itself isn't the best place to pull back the curtain, at least not all the way and certainly not every time.
*Unedited, pre-layout, and truth-to-tell, working version of the monster. The name could change, or for reasons, we might decide this monster shouldn't go in the Bestiary.