We're not talking about mundane Shell Shock here, that'd be the trope called Heroic BSOD. Instead we're talking about a full-fledged Freak Out, or possibly even as bad as a Face Heel Turn. If you Go Mad From The Revelation, it'll be some sort of Homicidal Mania or at least Schizophrenia, and sometimes even Permanent Catatonia. A really lucky PC might get away with just an irrational Phobia, but the NPCs should almost always get much worse than that.
Worse yet, this sort of insanity generally isn't caused by "mundane" explanations like battlefield stress, extreme chemical imbalances, or demented upbringing. It comes at you because you saw, or maybe just contemplated, something Man Was Not Meant To Know. May have only been glimpsed for a moment, but you'll feel the effects for the rest of your life. Since the cause was otherworldly, modern Psychiatry is probably unable to help in any meaningful way.
This can also be a side effect of tripping a brain mine.
The proliferation of this trope can be largely attributed to the works of H.P. Lovecraft.
Game and Story Use
- Many RPGs have mechanics to cover exactly this sort of thing. Call of Cthulhu and Trail of Cthulhu have extensive systems for it, but even some less Lovecraftian games, like Savage Worlds have just such a thing.
- You might surprise your players by porting such a system over to your game of choice, and using it when your (thus far only mildly creepy) scenario builds to The Big Reveal.
- Justifies (well, at least enough for fiction and gaming) the sick psychosis of any villain short of the Big Bad Evil Guy. Unless the campaign is low key and/or very realistic, the BBEG is probably be the type that drives others insane, not the type who is insane.
- If you fancy reducing the incidence of magic in your campaign world, this can be a good way of doing it - the more you know about the fundamental forces underlying reality, the madder you become.
- This can also appear as a side effect of "wishing for infinite knowledge" or getting a crossed line on a high powered divination spell … you may not exactly go mad, but the volume of information crashes your brain instead and leaves you far less than you were.