Terrigenesis

"Every cell within him has changed, in his body and his mind."

- Jiaying about Gordon

Terrigenesis is the process that activates certain Inhuman genes, allowing the achievement of superhuman abilities after exposure to the mist produced by the Terrigen Crystals.

Origin and General Application
"Perhaps you know the tale of Terrigenesis." "Of course. Ancient Kree descending on planets, altering its inhabitants to fight in their war."

- Vin-Tak and Sif

Terrigenesis is a process developed by a radical faction of the Kree race known as the Reapers,  during their experiments to create living weapons to serve as new soldiers in a prolonged war. By performing numerous genetic modifications to various intergalactic races, the Kree planned to expose individuals to the mist produced by Terrigen Crystals, sparking the Terrigenesis process, which would manifest diverse biological changes and powerful abilities that could be weaponized. On Earth, the Kree were forced to abandon their experiments when Alveus and his army led a rebellion against the Kree. However the Kree left behind their other test subjects, who would become the Inhumans, and several Terrigen Crystals that they had hidden away. Centuries later, a small fringe society of Inhumans had gathered together in China's Hunan Province. Within this society, certain members were allowed to undergo Terrigenesis to obtain their hidden gifts when they reached a certain adolescent age, an age when their elders believed they had developed enough to endure the changes, but were still young enough to adapt to it. One Inhuman, Jiaying, who had received extended longevity through Terrigenesis, was charged with guiding the generations of newly emerging Inhumans through their changes and help them harness their new abilities. One of these new Inhumans was Gordon, who emerged from Terrigenesis with the power of teleportation, but lost his eyes, with a layer of flesh covering the area where his eye sockets once were. Gordon eventually grew to master his powers and took upon himself Jiaying's responsibility of guiding new Inhumans as she had once guided him.

Within the settlement, named Afterlife in English, newly transformed Inhumans would be given a form of acupuncture involving small, shining needles that helped the body to adapt to the thousands of years of evolution which had taken place within moments. The "transitioner", the person who gave the acupuncture, would later train the person in how to use his power.

Skye's Transformation
"How do we stop it?" "I don't think we can."

- Antoine Triplett and Skye

Skye entered a chamber in the temple of the Kree City to stop Raina, but she arrived late, as the Diviner Raina was holding glowed and floated onto a pedestal. The entrances to the chamber started to close and the Diviner opened, revealing Terrigen Crystals. The crystals grew and released the Terrigen Mist, which caused Skye and Raina to form black, goo-like cocoons and to petrify. After a few seconds the cocoons crumbled, revealing Skye and Raina had unleashed their Inhuman potential. Jemma Simmons did a chemical analysis of Raina's blood; she found that Raina's DNA was altered on a molecular level with the addition of macro-molecules. Leo Fitz analyzed readings from Skye's heart monitor and discovered that during her transformation, she had a heartbeat of 300 beats per minute, a number he called "inhuman".

Other Inhumans
"I felt different, like, like a part of me was unlocked."

- Andrew Garner

During the War against the Inhumans, Skye used her powers to push a Quinjet filled with artificially-created Terrigen Crystals into the sea; these crystals dissolved in the water, releasing Terrigen Mist, which contaminated the wildlife and were eventually turned into Nature Max Supplements Fish Oil pills which were distributed worldwide. Taking these pills caused other Inhumans to undergo Terrigenesis. One example was Joey Gutierrez; inside his apartment in Seattle, Washington, Gutierrez emerged from a cocoon with the ability to melt metal in his proximity. Meanwhile, Andrew Garner received the property of Jiaying for analysis; inside a ledger was a booby-trap that released Terrigen Mist, intended to kill any humans who attempted to read the books. Garner was caught by the mist and Terrigenesis occurred; although he initially showed no signs of physical change, once near another Inhuman Garner transformed into a large grey skinned monster who desired killing other Inhumans. Garner gained the alter ego nicknamed Lash.

Jiaying kept a man known as James from undergoing Terrigenesis because her tests deemed him unworthy for powers; Hive, on the other hand, needed to be able to possess his knowledge, so he exposed James to the Terrigen Crystals, changing him to Hellfire. The newly-transformed James was quickly possessed by Hive while he was learning control over his powers.

New Experiments
Shortly after swaying James, who soon began calling himself Hellfire, Hive recruited Dr. Holden Radcliffe to create more Inhumans by recreating the Kree's experiments in Union City, Wyoming. At first, Dr. Radcliffe attempted to use Hive's blood to recreate the experiments, but it failed to work on the HYDRA volunteers, liquefying them as they screamed in pain. Realizing the formula specifically needed Kree blood, Hive then attempted to capture the two Kree Reapers and use their blood for the experiments. Unfortunately, Hive consumed one and the other was destroyed by S.H.I.E.L.D. agent Alphonso Mackenzie's Splinter Bomb.

Despite the setback, Hive and Dr. Radcliffe continued the experiments, using enthralled S.H.I.E.L.D. agent Daisy Johnson's blood; Johnson had previously been injected with a drug derived from Kree blood. Needing new test subjects, Hive and Hellfire then captured a group of Watchdogs for the experiment. This time, it was successful, but the Watchdogs were reduced to hideous-looking, barely sentient beings quickly dubbed Primitives.

A Long Transformation
"It's incredibly rare to see the actual metamorphosis; it only lasts a few fleeting moments." "Not this one. The subject's been gestating for seven months." "Come again? How has he survived?"

- Jemma Simmons and a scientist

Senator Ellen Nadeer held in her house her Inhuman brother who had been undergoing Terrigenesis for months. After a deal she made with S.H.I.E.L.D. Director Jeffrey Mace, Nadeer brought the Inhuman to a government lab and asked Mace for the help of his agent, Jemma Simmons to free the Inhuman from his shell. Simmons managed successfully to break the Inhuman from his shell, but was immediately taken away from there.

Process
During Terrigenesis, the host becomes enveloped inside a stone-like "cocoon", where their powers can develop. After the development phase, the cocoon then shatters. The hosts always emerge with some kind of power, mostly to fulfill a purpose. Some Inhumans who emerge from Terrigenesis may have physical changes to their bodies (for example, when Gordon emerged from his cocoon, his eyes have disappeared).

Trivia

 * In the comics, the Inhumans going through the process of Terrigenesis form an actual, insect-like cocoon, and they can undergo Terrigenesis multiple times.
 * Vijay Nadeer is the first known Inhuman in the Marvel Cinematic Universe to undergo Terrigenesis more than once.
 * Apparently, the process of Terrigenesis causes discomfort to those who undergo it (Elena Rodriguez described it as "rock crawling up [her] body").