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.