Mana is a magical energy that exists in special individuals who are able to manipulate it into creating spells. These individuals are referred to as mages and are valuable assets to any nation that employs their services. This awards them a number of priviliges in society. However, their greatest strength also makes them vulnerable to some environmental factors. There exists pockets of areas called dead zones, in which large amounts of the magical energy coalesce. This raw Mana permanently settle in an area, creating various magical effects.
It is a significant problem to perform spells within or even at the edge of these areas. For those who manage to create magical effects, it becomes very difficult to control their energy. Using Mana in these areas is dangerous and potentially fatal, and spells can backfire on its user (or: fireball spell that sets the user on fire). These dead-zones are also the cause of naturally forming magical monsters. Creatures that live inside a dead zone have a high chance to produce mutated offspring who are able to innately manipulate mana and perform sorcery. Fire breathing dragon's, flying sphinxes, minotaurs, and other dangerous magical beasts or chimeras develop and oftentimes travel outside these dead zones to spread havoc among villages and cities.
As mages, the most powerful weapons of a nation, are powerless to go into these sites and kill these beasts before they become a threat, a kingdom must depend on mercenaries to venture into these areas and deal with it for a few. How is it that these magical creatures are able to manipulate Mana even after leaving their zones, while mages are not when entering them?