The Terrigen Mist is a gaseous substance that is produced by Terrigen Crystals. When an Inhuman is exposed to the Mist, they generate a rocky cocoon and undergo Terrigenesis, developing superhuman powers.
In order to change the DNA of Inhumans and give them their abilities, the Kree created the Terrigen Crystals which released the Terrigen Mist. Once an Inhuman was exposed to the mist, they would go through the Terrigenesis and become fully Inhuman.
When the Diviner opened to reveal the Terrigen Crystals inside, the Terrigen Mist filled the temple of the Kree City and surrounded Raina, Skye, and Antoine Triplett, who watched as the women were seemingly petrified upon exposure to the Mist. Triplett kicked the crystals, causing them to shatter. When the Mist cleared, Skye shattered her cocoon along with Raina.
Artificial Terrigen Crystals were created by scientists on the order of Jiaying, so that if the Diviners were lost or destroyed, the Inhumans would not go extinct. Terrigen mist surrounded Robert Gonzales when Jiaying used an artificial terrigen crystal to kill him during her Ambush in Afterlife. 
During the War against the Inhumans, Jiaying had a team of Inhumans led by Gordon hijack the S.H.I.E.L.D. Aircraft Carrier, which sent out a distress beacon. In order to convince Anne Weaver to broaden the beacon to set a trap for S.H.I.E.L.D., Jiaying broke a terrigen crystal in a sealed room containing Agent Oliver and two other colleagues; Oliver and his colleagues choked on terrigen mist as their bodies turned to stone.
Skye used her vibrational powers to send a Quinjet filled with crystals into the ocean. At the ocean's bottom, terrigen mist mixed with the water and the wildlife; the fish were caught and turned into Nature Max Supplements Fish Oil pills that were distributed worldwide.
With Jiaying dead, Phil Coulson recovered her records from Afterlife and sent them to Andrew Garner to study them. As Garner opened one book, Terrigen Mist hidden in the book as a booby trap enveloped him and he underwent Terrigenesis.
The mist is released by the Terrigen Crystals and has no effect on any humans who are exposed to it. However, whenever the mist is exposed to latent Inhumans with the dormant genetic potential, the Inhumans begin to undergo Terrigenesis. The mist forms a rocky "cocoon" which envelops the subject and shields them from the outside world, allowing them to transform physically or manifest their powers.
However, when the Afterlife Inhumans began artificially producing Terrigen Crystals in order to preserve their heritage, the scientists were unable to separate the metal of the Diviner from the crystals. As a result, the altered mist was lethal to humans due to their incompatible genetic patterns. This modified mist would still induce Terrigenesis in potential Inhumans, but upon contact with humans, the Diviner metal would slowly petrify and gruesomely kill them. 
|Appearances for Terrigen Mist|
In chronological order:
- ↑ Agents of S.H.I.E.L.D.: 2.12:
- ↑ 2.0 2.1 2.2 Agents of S.H.I.E.L.D.: 2.10: What They Become
- ↑ 3.0 3.1 Agents of S.H.I.E.L.D.: 2.20: Scars
- ↑ Agents of S.H.I.E.L.D.: 2.22: S.O.S. Part Two
- ↑ Agents of S.H.I.E.L.D.: 3.07: Chaos Theory
- ↑ Agents of S.H.I.E.L.D.: 3.18: The Singularity
- ↑ Agents of S.H.I.E.L.D.: 4.13: BOOM