A product of Kreen genetics research. Derived from 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.