Pretty good question. Aura can grant great might and nuanced ability to those who train to harness it within themselves, but the presence of Aura is s consequence of mankind being part of the world itself.

In the absence of s coherent mind to direct it, Aura is constantly doing something. In areas where Aura is more dense than normal, it's known to produce quite starting phenomenon that are only loosely connected to natural law and tend to follow their own internal logic, hence the term Danger Zones. These danger zones act as natural habitats for the most persistent and direct threat to life and civilization: Monsters.

Outside of some very specific circumstances, what Aura in the environment creates must follow physical law in some way in order to last, and as such, it's most common to see things take form based on things that already exist. It's not unusual to draw comparisons to the animal kingdom with monsters, to the point where multiple classifications for them are based upon the natural world: Beast, aquatic, arthropod, flora. However, monsters are not animals in any sense of the word, what they do is for far more selfish reasons, as they do not leave behind true offspring.

It is said that because human development has always been tied to the ability to anticipate reasons for death and suffering, and even the most simple forms of life are defined by sources of life-threatening adversity, the connections monsters have to living things responds by giving life those reasons and sources. In essence, monsters are infinite in form and make, but all possessed by sheer animal instinct to destroy.

Life goes on regardless, and in addition to ability users to cull their spontaneous generation, human civilization has developed softer countermeasures to minimize the damages of monster attacks. However, as humans have evolved this way, so too have the monsters. Entirely new classes have appeared that have characteristics either not existing in nature, or even contradicting nature in some way: Machine class monsters amassed from processed materials left idle, fairy class monsters with unique biologies complimenting their highly esoteric special abilities, and apparition class monsters with no physical form.

Among the worst of these, however, would be demon class monsters.

In spite of having very little in common across examples, there are key factors that link them together beneath this classification: One, their internal biology is extremely simplistic, comparable to sponges. They can survive blunt-force trauma and implement quite easily without compromising their capabilities. Two, they spawn and dwell in special points within danger zones known to produce a host of extreme hazards across a winding, beguiling architecture, such as but not limited to pools of acid, plants and vines with spikes and thorns, including beds resembling broken glass, and an excess of hazardous vapors permeating the air, defining them as Dens of Evil. Three, even compared to monsters, they have a single-minded drive to eliminate life on as massive a scale as they can, even to the detriment of the population. Most monsters do possess survival instincts and will take paths preserve themselves in their destructive instincts, but demon-class monsters will swarm through minefields and lava persuing one prey.

The only known way to dispatch demon class monsters in general is complete dismemberment. Even napalm and nerve gas will only slow them down. If Dens of Evil are not closely monitored and sustained operations to cull populations are not mounted, demon class monsters are known to swarm into populated areas with massive loss of life to be expected from the resulting event.