The Terrigen Mists are a vapor produced by the Terrigen Crystals. The crystals were discovered by the Inhuman geneticist Randac around 25,000 years ago. Randac exposed himself to the Mists giving himself incredible mental powers. Because Randac wished all Inhumans to undergo the Terrigen Mist treatment (a process known as Terrigenesis), the majority of the Inhuman population became deformed monsters. After centuries of experimenting with birth control and genetics, the Inhumans have been able to expose themselves and not become monsters. However, even after all these centuries, it has not been perfect. Some Inhumans that were exposed became devolved, mindless beings. They were called the Alpha Primitives and were used as slaves. As a preventative measure, all Inhumans are subject to genetic screening before exposure to the mists in order to ensure compatibility with the process. Terrigenesis has became a sacred ritual of the Inhumans, and a cornerstone of Inhuman Culture.
Approximately 100 years ago the Inhuman scientist Ultarnt was commissioned by the Genetic Council to do a study on the nature of Terrigenesis. The results of this study could result in a deeper understanding of the Inhuman rite of passage. It was learned that the process was not random as has always been seemingly apparent, but instead follows a greater and possibly discernible pattern. It was identified that in times of climate change Terrigenesis would favor mutations that would allow Inhumans to adapt. In times of war or conflict other appropriate mutations would arise. When an integral member of Inhuman society would die a replacement mutation would occur within a matter of three years. The far-reaching implications of this study were deemed too provocative for the general public and as such have been kept secret from most of Inhuman society, including members of the Royal Family.
When the mutant Quicksilver stole the Terrigen Mists and Terrigen Crystals the crystals eventually ended up in the hands of ONE, who refused to return them to the Inhumans. After this the Inhumans declared Silent War upon the Human race, particular the United States government.
Exposure to the Mists
The process of Terrigenesis can be extremely dangerous despite rigorous control on the part of the Genetic Council. Mutations very greatly in function and level of power. Mutations vary in every way imaginable including energy forms, psychic mutations, plant and animal control, sonic control, healing, death, strength, blast powers, flying, elemental manipulation, and much much more. Some of the least powerful Inhumans can do little more than change skin pigment. Some of the more powerful can level mountains or more. The most famous and powerful inhuman to use the Terrigen Mists is the Beyonder, though it is reasonable to believe that this was simply The Beyonder manipulating the illuminati and playing tricks on them.
For further reading see: Terrigenesis