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Go Mad from the Revelation. It sounds like when you learn something that drives you insane, but by the description, it can just as easily apply to something that drives you insane just from perceiving it.
Go Mad From the Revelation does also say this:
"There is generally a distinction between things that happen to the mind because of experience and things that are done to the brain. This trope is the former."
So it would make a difference whether a character just observes Phlebotinum doing something strange and unnatural, or if the Phlebotinum does something strange and uunatural to him or her, especial if he or she is unaware of it.
Oh, yeah. The article explicitly includes the passage: "Thus, insanity caused by drugs or a specific, quasi-magical effect (like a Brown Note) doesn't qualify. Contrast those things with the Shoggoths, who strain people's sanity in spite of never having that as a stated special ability — the thought of them is just that horrible."
So a magic power that is defined as "makes people go insane" doesn't qualify, but if an entity or object has a quality — such as Alien Geometries — that, as an unfortunate side effect, makes you go insane when you perceive it, then it would qualify.
In other words, something designed to drive people crazy probably wouldn't be this trope - but anything that, by its nature, makes you go crazy when you perceive it, most likely would.
To sum up,
- A Brown Note is phlebotinum that causes adverse effects as a primary function (or at least one of its major functions), such as causing insanity or physical harm. The banshie's wail in some variants of the myth, for instance.
- Go Mad from the Revelation is this for something so horrible or incomprehensible that it breaks your mind simply by the fact that you know it exists. Eldritch Abominations, Alien Geometries, Tomes of Eldritch Lore tend to do this a lot.
^^ Scorpion451 did a nice job with that distinction — so depending on the specific situation, what you're describing could be either, but it sounds like it's probably more like a Brown Note.
Phlebotinum that causes people to go insane when exposed to it. Basically Phlebotinum-Induced Stupidity, except it makes them crazy. Very common in science fiction/horror stories.
Edited by FossilsDaDaDa