Mist mutates wildlife into monsters, such as in the Evil Forest. Thus people build their cities on the plateaus above the Mist, though the world outside of settlements remains dangerous even on high ground. Most avoid traversing the monster-infested and perpetually dark surface of their continent, opting to cross the Aerbs Mountains via cable cars, or using Mist-powered airships developed and built in Lindblum. As traversal outside of Mist Continent via airship is not viable, the rest of the world remains largely unexplored, as it is still mainly free of Mist.
The Mist affects the environment in various ways. It blocks out sunlight, making the affected areas murky. Chimomo the moogle at one point complains that fishing has been bad since the Mist appeared, and another moogle, Mocha, complains that their old records are fading because of the Mist.
Mist is residual element of the souls that should return to the planet, but is discharged into the atmosphere instead. According to the Final Fantasy IX Ultimania, Mist appeared around 800 years ago. Garland, the leader of Terra tasked with overseeing its overtake of Gaia, unleashed the Mist upon Gaia to disrupt its people so that Terra could one day be reborn.
Mist is distributed through the Iifa Tree's roots and is, in fact, the Gaian souls being refused to return to their crystal and expelled to flow back into Gaia's atmosphere instead. These stagnant and tormented souls are reborn as monsters. Mist is found primarily on the Mist Continent, where its effect causes many of the kingdoms to retreat to higher elevations. The power of the Mist hardens hearts, provoking conflicts around the continent.
Mist Continent denizens, and in particular the regency of Lindblum, have developed Mist-powered engines, which are primarily used to power airships. Airships will thus only function where there is Mist, meaning they can't fly outside of the Mist Continent. Additionally, the cable cars within the Aerbs Mountains are also Mist-powered. The advent of airships brought the wars to a halt, and Garland decided to create a Genome that would incite war on the Mist Continent instead. Garland's first creation for this purpose, Kuja, began to plot against him, and taught Queen Brahne to manufacture black mages from the Mist. Kuja also claims he can manifest Mistodons from Mist on a whim.
After Zidane and his party defeat the Soulcage at Iifa Tree's base, Mist production stops for a time. Soulcage's purpose was to act as a "soul divider", rejecting Gaian souls while allowing Terran souls to circulate, and with its death, Iifa Tree stops expelling Mist through its root system. However, after Kuja discovers his Trance powers and destroys Terra, Iifa Tree activates and expels Mist onto the entirety of Gaia, its roots violently breaking through the planet's crust, causing earthquakes all around the world. After the Iifa Tree settles following Kuja's defeat, the world is finally freed from the scourge of Mist. Even if the Mist is gone, Vivi was still able to build new black mages, dubbed "Vivi's sons", even though black mages were said to have been produced using Mist as an ingredient.
Location backgrounds change depending on whether the area is covered in Mist, but this is only a cosmetic change; though the Mist is said to have an effect on monsters, random encounters stay the same in both Mist-filled and Mist-free versions of a location. The world map enemies change in the endgame, however.
In practice, the world map has two settings: with Mist or without. If one were to use cheats to travel outside the Mist Continent in the beginning of the game, they would find that the Mist effect covers the globe. When the party emerges in the Outer Continent, the Mist effect is gone from the world map until after they return from Terra, during which the globe is again covered in Mist. Not every location has differing backgrounds, e.g. Chocobo's Forest.
The field locations that have two versions of their backgrounds because of Mist are:
- Aerbs Mountains gates and mountain path
- Observatory Mountain
- Lindblum Castle lookout point and port
- Iifa Tree
- Madain Sari
- Ipsen's Castle
Kuja claims to manifest Mistodons from Mist. They are fought in locations Kuja sets out to attack: Iifa Tree during the First Battle of the Iifa Tree, and in Alexandria during Battle of Alexandria. In the endgame, Mistodons can be randomly encountered on the world map, no longer manually created by Kuja, but seemingly manifesting from the Mist on their own.
- Final Fantasy IX Ultimania, p.44–46