Miasma

Miasma (瘴気) is a highly corrosive and poisonous gaseous dark purple mist that originates from Cracks in Mass for the Dead.

Overview
A mist that springs from the depths of the abyss of Cracks. It has a corrupting effect, causing animals and plants to go wild, creating Contaminated Beasts. Even the smallest particles that are invisible can still have an effect. Those who have some power in the New World are not totally immune to its corruption, though the level of tolerance depends on the individual.

The gas coincides when a Crack spews out Chaos Beasts at regular intervals. The substance is only dissipated once all the Chaos Beasts are defeated. However the contaminants contained within the miasma have been shown to not just affect fauna and living things, have some long-term effects on local flora and the earth, creating bizarre creatures.

Trivia

 * However, there are no cases where the people of the Great Tomb of Nazarick have become violent, including the protagonist who has no combat ability.
 * No cure for the miasma exists so far, however, efforts by both the Pharmacist's Guild and Nazarick are underway to develop one.
 * Gazef Stronoff was infected by the gas, however, unlike most cases where monsters go feral, he was still able to retain his warrior instincts and utilize his Martial Arts.
 * The abnormal changes that subjects undergo once afflicted by the gas, is later revealed by Sophie Noia to not just be a poison but to also radically change the subject's race, being akin to a "curse". Attempts to resurrect subjects to restore them to their original selves have only shown that they remain in their contaminated states even after revival.