Final Fantasy Crystal Chronicles
Miasma is a gaseous substance which plagues the world in Crystal Chronicles. Such is the nature of the substance, people cannot be exposed to it and have had to find ways to protect themselves as even minimal exposure can lead to instant death.
The purpose of the Miasma is to cause pain and suffering to those who inhabit the world, as Raem (the main antagonist in Crystal Chronicles) prefers to devour memories of this nature.
Miasma Breeds Monsters
Miasma is the cause of monsters in the world and many theorise that before the Miasma came into existance, there were no monsters around to terrorise people. It is said that Raem uses the monsters, alongside the fear of impending doom, to further increase the level of suffering that people have to endure.
Although these monsters rarely attack established dwellings, they make it very difficult for Crystal Caravans to obtain the Myrrh that they need to maintain their village's defenses.
Crystals Provide Protection Against Miasma
Crystals effectively provide a shield from the Miasma, with bigger crystals providing much more area for the shield. Unfortunately, they can only provide shielding for a limited period of time, and Myrrh must be used to replenish their protective power.
When Crystal Caravanners trek around to source out Myrrh, they too carry around a miniature crystal on board their Crystal Chalice. This allows them to cross through Miasma Streams if they have the correct type of Myrrh.
Final Fantasy Crystal Chronicles: My Life as a King
In My Life as a King, King Leo's Kingdom has been destroyed by Miasma. He starts out on a quest to try and revitalise his Kingdom by finding sources of Myrrh in order to build a protective shield.