Mind horrorism is belief in mind horrors, and is what makes mind horrors exist.
Mind horrors are a particular kind of horror that is believed to come from another plane, the Void, which can enter the real world (the world of Faith) when a person works memory spells (or other belief-altering spells), and upon entering reality, it proceeds to consume the spellworker. It is also believed that any means of creating altogether new sentient minds - other than via the natural means (pregnancy and childbirth) - will also attract mind horrors. The end result is to make it very dangerous to create minds that could believe strongly in their own magical ability (as this is oftentimes what the creation of sentient minds is usually done for).
Mind horrors do not attack every single time a person works memory or mind spells, rather the chance increases with "greater" memory or mind spells, primarily understood to mean such spells that make the subject believe more strongly in their ability to work spells and overcome opposing spellworkers; and working more of these spells. Hence, no amount of, working a series of memory or mind spells without incurring a visit from a horror, is proof against the horrors' existence or interest.
The history of the belief in horrors can be traced back to the times of the Memory Revolution. When memory magic was discovered to be able to grant the memory-subject powers greater than had ever been seen before, it was claimed that memory spellworkers would usher in a new age of magical power for everyone. However they were quickly corrupted by their greed for power, and in the process of empowering their subjects they enslaved their subjects to their will, resulting in one of the greatest cataclysms the world had ever seen, the Memory Revolution.
A method had to be found to prevent the memory spellworkers from empowering more subjects. The Order of Horrors founded with the task of creating a meme telling of the dangers of calling horrors, and of spreading this meme to all the world so that the memory spellworkers would receive retribution for their spells. Horrors were intentionally designed as an ideological weapon against all memory spellworkers. As horrorism (belief in horrors) was intentionally created, it is a manufacted faith.
Resistance and Consequence
Because there are no good descriptions of horrors, as very little is known about them, and so it is practically impossible to defend against an expected horror attack. Memory spellworkers tried to imbue memories of their memory-subjects, of successfully defeating these horrors; however, the lack of a a clear description of what a horror looks like means that they are not able to input memories of fighting a horror, only memories of fighting something that might look like a horror but might be totally off the mark. Additionally, any memories of fighting and defeating a horror only means that the subject had defeated a false horror, by definition, and that they have not encountered a true horror. This essentially makes it impossible for a memory spellworker to create memories to boost confidence against horrors (boosting confidence against opponents is the primary benefit of memory spellworking).
As horrorism spread, the memory spellworkers tried to empower their subjects against them, to no avail. One by one the memory spellworkers were beset by horrors attracted to their particular form of magic, and devoured. Their attempts to work even greater memory spells however attracted more and more horrors, which all but annihilated their ranks, and their stalwart spell-battles against the horrors all resulted in one-sided massacre. As the memory-subjects were the most powerful spellworkers in all the land, the ability of horrors that could annihilate them with ease quickly made these creatures feared by all. With the collapse of the memory spellworkers, the empires of memorism (alteration of memory) came to an end, and the practice of implanting memories became forbidden magic, to use only at one's peril.
In current times the Order of Amnesia only very rarely puts in memories out of fear that they would be made a horror's next meal.