This strange energy is given off by horrors and causes intense reactions in any organisms that are exposed to it, mutating their bodies wildly. Like radiation, objects exposed to it can retain traces for a long period afterwards, making it difficult to deal with.

Common mutations include an increase in size, extreme hunger and difficulty controlling emotions. Additional eyes, teeth and claws are also likely, with many more changes possible depending on the species being mutated. Individuals that become visibly mutated are known as monsters, and can be either welcomed into society or ostracized depending on location and the severity of their mutations.

However, as a creature is exposed to more horror radiation, mutations will become more extreme, eventually leading to the creature becoming a terror, a feral and dangerous monster that is driven to do little but consume what it can reach and breed with more of its kind. Terrors do not typically give off horror radiation, though there have been known to be unfortunate exceptions.

Despite these downsides, research has been done on ways of controlling mutations arising from exposure to horror for clinical or aesthetic purposes. Low-level radiation over a long period of time has been found to have much more benign and directable effects than the high-level exposure most beings who come into direct contact with horrors experience, and is intentionally performed in certain locations. Populations living near vents that connect to horrors underground can slowly mutate over the course of years and generations, with little in the way of harmful consequences.

As well as creatures being vulnerable to horror energy, plant life is also affected, though usually with far less extreme results. Constructs can be resistant, depending on the specific make and model, while more esoteric entities such as gods, angels, and fiends are almost always fully immune.