A product of Kree genetics research. Using stolen Skrull genetic manipulation therapies, a Kree science colony on Earth was seeking a way to trigger the latent Celestial DNA encoding in humans and activate their metagene. A gas delivery system was selected for most efficient exposure.
Exposure to the Terrigen Mists begins the process of allowing Celestial helicase enzymes to unzip DNA helices and rewrite them with the ancestral Celestial genetic coding for metagenes. If no ancestral Celestial DNA is present in the subject, the result is universally fatal.
The results of Terrigen exposure are unpredictable. More common successes include vastly prolonged lifespans and physical capability. A very wide variety of mutations have been observed, particularly due to the genetic drift in humans in the 50,000 years after the Mists were first manufactured.
Research and experimentation on less-evolved lifeforms was forbidden by mutual compacts signed by not just the Kree Empire but also Oa and the Nova Corps. With the Omrut Beacon believed to be destroyed, the Kree were unaware that the Terrigen Mist project was successful or that it had latter-day applications.
The Atillans maintained the only supply of Terrigen Mists after the Kree departed Earth. When the White Martians invaded Earth, they used the Terrigen Mists on the Omrut Beacon to delete the Celestial encoding in 99.9% of human genomes. The Atillans spent generations afterwards trying to identify bloodlines that resisted the White Martian manipulations. They started breeding programs designed to bring compatible humans together and combined mystic esotericism with the genetic manipulation of the Mists. Atillans took on positions as gurus, mystics and holy sages, and used mystical cults and religious trappings as cover for their long-term efforts to locate potential future additions to the Inhuman contingent.