Called horrorism is belief in calling horrors, and is what makes horrorcalling possible and giving rise to these horrors.
Horrorcalling is the process of calling a horror, and can be done via a ritual (depending on regional beliefs, this could be involving spilling of blood, arcane incantations, use of gems or requiring death sacrifice, etc) by an individual or group (as long as they believed in called horrorism). The ritual would call a horror and direct it to a particular location, and it would then proceed to attack any spellworker at the target location for a short while before fading back to the Void. However, there is a significant chance that the ritual would just call the horror directly upon the ritualists, (who by working this ritual are also spellworkers), which would spell their death, making this a dangerous endeavor to only be used in the most dangerous of times, and rarely to be used for just personal gain or vendetta. It is understood that the ritual may call false horrors as well as true horrors, so it is not guaranteed to be successful (hence when it fails it does not disprove the belief itself). Note that horrors have no interest in people who cannot work spells.
As horrors are believed to be extradimensional creatures that are impossible to comprehend and nigh impossible to defend against, and with the ability to horrormark anyone they come across, they can be relied upon to finish an enemy spellworker, even a very powerful one, as they cannot readily defend against the horror and even if they succeed, they would have only defeated a false horror, and not a true horror, and other horrors would still be attracted to the spellworker’s horrormark.
Horrorcalling was designed to counter megalomaniacal spellworkers who would otherwise reign unchecked over other spellworkers through the sheer extent of their narcissism (which empowers them more than others’). Any spellworker who sufficiently abused their powers or were seen as significant enough of a threat became subject to a horror attack, and the threat alone would force such individuals to always keep on the move (so that they can’t be targeted) and to keep a low profile (so that none would want to target them).