The First Madness
Avool pan Ankhri was a failing con artist. He made a living by selling snake oils, false cures, and famously--pamphlets. He worked out of Shar, and could often be seen on the streets selling his worthless potions and false charms against the evil eye. He also sold booklets of pseudo-religious nonsense: The Secrets of the Celestial Martyrs, The Hidden Prophecies of St. Nashanial, and The Five Forbidden Pressure Points of Dreadful Death.
After a particularly bad day, involving the collections of debt collectors, the scorn of the crowds, and the infidelity of his mistress, Avool sank into a particularly black mood. He retreated into his wagon to write what would be his last pamphlet: The True Nature of our Situation. He took the pamphlet to the printer's and had a few hundred copies made. By the next day Avool pan Ankhri was selling them in the marketplace, and the printer was found hung from the rafters, his heels swaying in the cold air.
The facts are these:
Anyone who reads The True Nature will fall into a deep depression, and eventually take their own life.
Anyone who is told enough about The True Nature will fall into a deep depression, and eventually take their own life.
Anyone who learns about the philosophies of The True Nature, even by scrying or oracle, will fall into a deep depression and eventually take their own life.
In this way, the poisonous words of Avool Pan Ankhri spread across Shar, as people shared the pamphlets and discussed it with each other. Mothers wondered what their child had been reading before he took his own life. Men tried to talk their friends out of suicide, only to find themselves quickly convinced enough to join them. Even when a state of panic over the number of suicides gripped the city, philosophers and mages continued to seek out the booklet and read it--perhaps not believing that words on a page would drive them to end their own life.
Avool was forced to flee angry mobs of people whose loved ones had committed suicide. He crossed the bay to the Seamount, and hid for days in the caves. It was said that he carved a sequel to "The True Nature" into the inside of the caverns there. But, he was eventually captured by Shar guards with their ears plugged to avoid hearing the toxic words that Avool shouted while they carried him back for trial.
(Almost) all surviving copies of the dread pamphlet were destroyed, and all talk of it was banned. This is the only act of censorship ever enacted by Sharland. Since it was not considered a crime to print a pamphlet that wasn't treasonous, the city decided to imprison him for life, despite the enormous public outcry. He is there to this day, in a soundproof cell. The guards stuff their ears with wax and try to avoid looking at any words that the madman might have scratched in the dust.
Interestingly, the copies of The True Nature that were studied (but not read) by mages revealed them to be nonmagical, and completely devoid of compulsion-type enchantments. Any compulsion that they created in their readers was simply by the power of the words alone. The only person who has read the pamphlet without incident is Lhasadet, one of the serypha and the head librarian at the Library of Kalamon. She agreed with the wizard’s diagnosis, saying that the pamphlet was “just lines of ink on a parchment.”
Avool pan Ankhri seems to have written the single most depressing, fatalistic publication in existence.
The Second Madness
Even after the Madness of Avool had driven many of the citizens to Shar to suicide, it was some years later when the true evil of what Avool had created was realized.
The pamphlets had not been completely destroyed, and they could be copied safely by scribes who did not read Common. A few individuals began using The True Nature as justification for very evil deeds (although some of them were surely lying) or saying that the pamphlet had made them do the terrible things that they did. While the average person would receive the impression that "My life is not worth living", especially wicked individuals would gain the doctrine of "No one is worthy of life". Presumably, this was the philosophy that Avool pan Ankhri himself endorsed as he disseminated his terrible words. A number of death cults include all or part of The True Nature their doctrines.
The Third Madness
It seems that a people who continue living after reading The True Nature, and dwell frequently on the teachings of Avool, sometimes undergo a very bizarre physiological change. Specifically, several purple worms appear in their brain.
In a world that believes in spontaneous generation, wizards agree that this phenomenon is no more remarkable than the known fact that maggots spontaneously appear in rotten meat.
The worms were called Third Madnesses, and as they grow in one's skull, they cause headaches, delusions, and blackouts. After being released from the skull (use your imagination), the worms can grow to enormous sizes. While the smaller ones are can be somewhat domesticated, the larger worms (30' or so) are incredibly aggressive. Some of the aforementioned cults raise the worms, releasing them when they grow too large to control, while keeping the smaller ones to guard their lairs.
At all life stages, each worm can speak, and has a vocabulary of exactly one word (presumably taken from The True Nature) that it continually mutters. A Third Madness can secrete a staggering array of poisons. Larger worms learn to fight intelligently and can explosively vomit their stomach contents, usually a foul mixture of broken bone and bile. Like the ideas that spawned them, the worms are distinctly non-magical.
I'm of the opinion that a truly evil book that is capable of warping an innocent brain and inflicting deformity-inducing curses should not be any less terrifying if it is read in a null magic zone.
If you feed a program the wrong data or return the wrong variable type, it sometimes collapses into loops that can corrupt or crash the entire program. This is what happens when a conscious being understands the ideas in The True Nature. Like a prion (a single molecule) that can crystallize huge tracts of your brain. Like a seed crystal that can turn a liquid to a solid without any magic needed. This is what an idea does to parts of some unlucky brain.
The corrupted brain bits get trapped in this cycle, and they unconsciously begin to use magic. This single, primordial spell turns nearby tissue into annelid germ plasm. Alternate personalities get trapped in PCs brains all the time in fantasy settings. This is like that--only its an brain subroutine gone rogue, an unconscious loop in the blind spot of the primitive brain.
This is something of a radical idea: that a "cursed" fighter could simply have a corrupted subsection of his brain that is subconsciously casting crude spells. This doesn't work in campaigns where spellcasting is strongly tied to intention and intelligence and nothing else. But everywhere else, I believe this provides a novel and dangerous sort of "curse", that has its roots in memory, subsists through biology, and only eventually grows into the realm of magic.
P.S. I have no idea what category I should file this under. I only placed it here because the PCs are most likely to learn about the Madness of Avool through a doomsday cult.