Miasma

Miasma is a deleterious invisible field that leaks into the world when magic is used. The original Ashenal mages used magic in small amounts, so it had no serious effect. Caer's Mistake released a huge amount of miasma, radically altering the world and its peoples. Ethrenal developed techniques to push the limit of magic usage with minimal harm to one's self. As these techniques became widely used, especially during the hematic wars, when the schismatic era transitioned to the xerothermic era as the planet became desert.

Miasma Sickness

Exposure to miasma causes a cluster of symptoms that are described as miasma sickness. Brief exposure to extreme levels could cause immediate burns and blisters, while prolonged exposure would cause gradual organ failure in addition to neurological changes, especially confusion. Miasma sickness is what caused the 8 betrayers to instruct the stewards of the world to protect miasma tombs against any penetration at all costs.

Life-debt Theory

Some mages theorized that Miasma wasn't really something on its own, but a negative amount of life energy. In order to maintain life energy as a conserved quantity, the energy necessary to cause a magical effect was consumed ahead of time, then this negative energy would cancel out life energy present in nearby living things.