Miasma is a recurring term in the Final Fantasy series. It is a poisonous, gaseous substance that plagues the world and surrounds everything like air.
Final Fantasy Crystal Chronicles
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 of the people in Final Fantasy Crystal Chronicles. 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
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. It is a remake of the River Belle Path from the original game right down to the area being filled with gas.
In multiplayer mode, a Yuke who returns after you clear 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
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.