Tales of Phantasia
Miasma first makes an appearance in Tales of Phantasia, in which it is referred to as the wind of Daemonium. It is a substance that affects both humans and Summon Spirits, and although humans can tolerate it to a degree, it affects the sanity of Summon Spirits. Miasma is responsible for driving the Sylph into a berserk state, resulting in the need for the game's protagonists to battle them as they climb the peak at Lone Valley.
Tales of the Abyss
Prior to the events of Tales of the Abyss, miasma first appeared during the Fonic War in B.D. 2699, when the use of powerful fonic weaponry placed a significant strain on the Planetary Storm, causing the planet's core to vibrate and emit the lethal, poisonous gas that eventually came to be known as the miasma. The war, in conjunction with the miasma, devastated the planet's populace, eradicating roughly half of the world's population. In B.D. 2709, the prophet Yulia Jue read the Planetary Score and predicted 2,000 years of history. She warned the warring countries the dangers of fonic weaponry and how the miasma would continue to spread as a result of their excessive use. Subsequently, the war ended that year.
One year later, with the land still greatly affected by the amount of miasma leaked during the war, Yulia, alongside her ten disciples, proposed the Float Project, a plan to raise the land utilizing the power of the Sephiroth Trees. When Yulia was arrested on a false pretense, the Float War began—a war waged by Ispania and Frank against those who endorsed Yulia's Float Project. When this war ended in B.D. 2713, Ispania and Frank emerged victorious, putting the Float Project into motion in N.D. 0001 (B.D. 2714), a new era. Upon raising the land, the countries' enemies were left in what became known as the Qliphoth, where the miasma still flourished. The new surface of the world eventually became known as the Outer Lands. During the course of the story, the miasma resurfaces, prompting the world and the game's protagonists to contrive a solution to the threat.
Tales of Xillia
In Tales of Xillia, miasma makes an appearance during the search to obtain the Devil's Arms at the Tatalian Abyss. In the dungeon, the protagonists discover a man surprised by the poisonous gas that has risen from the floor cracks. Milla Maxwell refers to the substance as miasma, a cursed gas that was sealed in the ground during the formation of the world. Prolonged exposure to miasma results in the death of any living creature. Miasma was originally sealed by the power of the Four Great Spirits during ancient times. There were four seals that correspond to each main element, but the seals of Fire and Water broke, so the miasma now seeps into the outside world. At the deepest level of the Tatalian Abyss, the group encounters Dranzort, the creature with Milla's weapon. After defeating it in battle, Milla restores the seal using the power of the Four and prevents further leaks.
Tales of Xillia 2
In Tales of Xillia 2, it is revealed that miasma is produced as a result of the Soul Purification Process operated by the Spirit Origin, playing a significant role during the events of Origin's Trial and serving as punishment should humankind fail the trial. As an agreement between the progenitor Kresnik, Origin, Maxwell, and Chronos, should humankind fail the trial, the Spirits will depart to the Spirit Realm, and the miasma produced by the Soul Purification Process will overflow the human world, thus reducing human souls into inanimate objects for producing mana.