What is Terrigenesis? It's a genetic transformation. It's a rite of passage. A sacred ritual. It's all of these things and more.
On the TV show Marvel's Inhumans, the Inhuman city Attilan (located on the moon) is home to the Terrigen Crystals. No one really knows how or why Terrigen Crystals form, only that they seem to form primarily in waters located deep beneath the surface of the moon. It's these Crystals, which are revered as spiritual artifacts among the residents of Attilan, that create the Terrigen Mists that give Inhumans their superpowers.
But let's back up for a minute. To fully understand Terrigenesis, you have to delve into the history of the Inhumans themselves. Inhumans are the product of genetic tampering millennia ago by an alien race called the Kree, whose ranks include Guardians of the Galaxy villain Ronan the Accuser, and who have a penchant for the philosophy of "survival of the fittest." The Kree took normal humans and experimented on them, creating an offshoot race with the hopes of this new species becoming powerful weapons they could control. But the Inhumans developed rapidly and became their makers' equals, if not betters, soon forming a society ruled by monarchy comprised of a king and/or queen and their advisors. They never felt at home among other humans, and feared and hated the Kree who made them.
One of their kings, an Inhuman named Randac, was the first to discover Terrigen Crystals. He recognized their potential for altering and enhancing genetic material, so he developed a process of exposing the Crystals to water at a very precise temperature to enable this. This process releases the Terrigen Mists, which then trigger a Terrigenesis mutation in any Inhuman exposed to them.
Randac became the first Inhuman to undergo Terrigenesis, stepping into the Mists, justifying it as a possible way of defending his people against the Kree should they ever return. His successful mutation of superhuman abilities encouraged him to open the process to others of his race. Unfortunately, he and his people quickly learned that Terrigenesis can cause superior enhancements or horrible deformities. Generations of these Terrigen-fueled mutations — both good and bad — caused the Inhuman gene pool to dilute. Screening tests were instituted meant to increase an individual's potential for a positive mutation — or detect and stop those who were more likely to come out deformed.
Something about inhaling the Mists unlocks the latent Inhuman genome present in all members of the species, activating the superhuman abilities that they always carried the potential for. Until recently (in the comics), access to the Terrigen Mists has been jealously guarded and closely monitored. While exposure was always a voluntary procedure, an Inhuman must pass through extensive examinations to ensure a positive outcome. Years of preparation are typically required before undergoing the ritual.
When the Mists are inhaled, an Inhuman instantly forms a cocoon, inside which the genetic transformation takes place. Anywhere from a few minutes to several hours later, the newly-mutated Inhuman emerges, forever changed. But this is just the most accepted method. Deviations from the norm aren't uncommon; Inhuman king Black Bolt was exposed to the Mists while in utero, for example, when his mother took part in an experiment. It's believed his pre-birth exposure is what gave him his Omega-level superpower.
At the end of a Marvel crossover event called Infinity, Black Bolt "accidentally" (but not really) let a bomb detonate in Earth's atmosphere. This was not a destructive bomb; it carried a concentration of Terrigen Mists. The Mists mingled with the air, spreading across the Earth and eventually forming into clouds that were pushed across the planet by natural winds.
It turned out, there were countless humans who carried latent Inhuman genes. Exposure to the Terrigen cloud caused these individuals to undergo spontaneous Terrigenesis, forming cocoons with no preparation and little warning. Overnight, the Inhuman population surged, and the royal family migrated to Earth to try and help these confused new members of their society.
On Marvel's Inhumans, it appears that the story is beginning at a time when the Terrigen Crystals are mostly still located beneath the moon-based city of Attilan. Its sister show, Marvel's Agents of S.H.I.E.L.D., has seen the rise of new Inhumans across the globe thanks to a storyline with similarities to Infinity.
On this show, some rare Terrigen Crystals have found their way to Earth. These new Inhumans living on Earth, like S.H.I.E.L.D. agent Daisy Johnson, have developed a subculture with rituals and beliefs of their own, which have some similarities to the practices of the true Inhuman society hidden away on the moon. But they're definitely not the same.
An accident took place on Agents of S.H.I.E.L.D. involving a container holding Terrigen Crystals being accidentally dumped into the ocean. Now, as in the comics, spontaneous Terrigenesis is possible all over the world. But these new Inhumans, like their comic book counterparts, are completely unprepared for the superpowers they suddenly possess.
It remains to be seen if the royal family, exiled to Earth in the pilot episode of Marvel's Inhumans, will encounter any of these new Inhumans borne from the events of Agents of S.H.I.E.L.D. If they do, Black Bolt, Medusa, Karnak, Crystal, and the rest may very well hold S.H.I.E.L.D. responsible for their part in the release of Terrigen Mists across the Earth. In a worst case scenario, it could even lead to war.