Miasma (瘴気, Shouki?) is a recurring term in the Final Fantasy series. It is a poisonous, gaseous substance that plagues the world and surrounds everything like air. Miasma is similar to Mist, a gaseous monster-creating substance found in Final Fantasy IX and the Ivalice Alliance.
Miasma is pure poison. Even the slightest contact harms a person, and prolonged contact with it can cause death. Furthermore, it is the substance that is responsible for the appearance of the monsters. It is said that long ago there was no Miasma and no monsters. Although, in Ring of Fates which takes place before the spread of Miasma monsters still exist. This is the same for My Life as a King where the Miasma is now gone but monsters still exist. Sometimes a random Yuke in My Life as a King will question this and say that the "definition" of what people thought monsters were will have to change.
The only way to keep the Miasma away is to remain inside the barrier of a crystal. The crystals in the world are protecting the people from the Miasma, but their power is not endless. Every year the crystals have to be cleansed by Myrrh, which is collected by the Crystal Caravans.
Miasma is the eternal threat to the people. Miasma is produced by the Meteor Parasite, brought to the world in a meteor that crashed into Mount Vellenge centuries ago. The gas brings pain and suffering to the world, so that the memories of the people are focused more on this pain and suffering. This is because Raem, who devours the memories of people, prefers these kinds of memories, and as a result has a somewhat symbiotic relationship with the parasite.
When the miasma began to spread over the land, the Carbuncles advised the inhabitants to live near the crystals, which emit a protective barrier warding off the miasma. As a result, settlements sprouted wherever such a crystal rested.
Final Fantasy Crystal Chronicles: Ring of Fates takes place before the Meteor Parasite arrived, thus before the spread of Miasma. In Alhanalem's collection of books, there is one book titled "CRYSTALS AND BIG BANG THEORY" which briefly reads, "As for the relationship between crystals and Miasma..." Not much is revealed by this quote, other than Miasma was known to exist before the Meteor Parasite's arrival. In certain dungeons there is a poisonous gas that fill rooms, similar to Miasma.
In the Crystal Temple, certain rooms are filled with the gas and can only be repelled by a Lilty's urn. When a Lilty's urn is placed on certain platform in these gas rooms, they create temporary sphere around the party that keeps the gas out just like the Crystal Chalice in the original game. The other dungeon filled with this gas is the River Belle, which is a remake of the River Belle Path from the original game.
In multiplayer mode, a Yuke who returns after the player clears a certain dungeon mentions that he seemed to wander into a separate world, filled with miasma, where the only safety was in the crystals' light.
Final Fantasy Crystal Chronicles: My Life as a King takes place directly after the original game. The young King Leo is beginning to restore his Kingdom after it had been destroyed by the Miasma.
While the Yuke that resembles Lich from Ring of Fates attempts to tell a moral story to a mother Lilty and her daughter, she makes a story about a disease in the air that slowly killed off people near it. Eventually, the Lilties believe in the story, with the mother Lilty stating "We're going home straight this instant! And when we do, we'll brush our teeth and scrub our hands until they're pink!"
The Miasma Woods connects the Harena and Florem Regions, and contains several portions of the ground that will poison whoever steps in it. Due to the skystone of the Eschalot being destroyed, the party is forced to travel to Florem by way of the woods. Fortunately, the awakening of the Wind Crystal allows the wind to pick up again, thinning the miasma somewhat and allowing the party passage through the woods.
Early medical theories predating the discovery of microorganisms often associating foul air, called miasma, from crowded conditions or rotten objects as a cause of disease.