|“|| Dr. Tsing: "Shouldn't you be in the dungeon with your monsters?"
Cage Wallace: "They're not monsters. They are soldiers. And the reason why you get to play doctor in your ivory tower."
|— Human Trials|
The Cerberus Project was the codename of the Mountain Men project to create subservient soldiers, known as the Reapers. The project was led by Cage Wallace, and was unknown to many in Mount Weather, aside from the Guard.
The process involved the use of the highly addictive Red drug in a brutal ordeal to turn the strongest captives acquired from Harvest Project raids on the surface. Once turned, Reapers were employed as outside guard and raiders to defending Mount Weather and bring back more captives in exchange for the Red drug.
The Harvest Project started 56 years prior when the Mountain Men realized the Grounders had adapted to the outside radiation, making them immune to it. Mountain Men were not able to adapt to the outside as they had remained in the Mountain the entire time. They eventually realized they could use Grounders to filter their blood of the radiation that was poisoning the Mountain Men. The Grounders, however, were presumably not willing to volunteer for this task so the Mountain Men would take them by force.
To make things easier, through the Cerberus Project, they turned the strongest Grounders into vicious monsters called Reapers in order to reap other Grounders into the Mount Weather Mines. The Mountain Men then inject the Red drug into the Reapers in exchange for Grounders. Once the Mountain Men had finished harvesting the Grounder blood, they would dispose of the dead and dying Grounders back into the mines for the Reapers to feast on.
Once the Grounders had been sorted into either Cerberus or Harvest, they would then go through decontamination where they are blasted with boiling water and doused with something that "burns even worse," according to Lincoln. They are given shots and pills as well as visually inspected before being placed into holding cages for later use.
Phase 1 of the Cerberus project had the subject securely strapped down on a gurney and injected with the Red drug every two hours while sounding a tone generator and accompanying it with electric shocks to the wrists for approximately 15 seconds.
Phase 2 commenced once the Grounder had become conditioned to show fear towards the tone generator (within a day or two). It involves two newly made Reapers pitted against each other to the death for another dose of the RED, which they become addicted to. The winner goes on to become a Reaper.
Once they have passed Phase 2, the newly created Reaper is then sent out to collect more Grounders in exchange for the Red Drug. Until Lincoln, it was believed that the conversion to a Reaper was irreversible.
Reapers are Mountain Men's primary force on the outside. Having been made subject to the harsh regime of Project Cerberus, these subservient warriors are are savage, primal, cannibalistic, and brutal, showing no mercy to their victims. Most Reapers appear disfigured, with horns and bumps across their faces and head. It is possible this is a side effect of the drug, Red, or some sort of body modification they participate in.
Once a Grounder has been turned into a Reaper, they are left in the mines to guard Mount Weather. As loyal guard dogs for the Mountain Men, the Reapers protect the mines as well as bring in new Grounders to the Mountain Men. The only food Reapers are given are the bodies of the Grounders that the Mountain Men have harvested of their blood. The Mountain Men use a tone generator which released a piercing sound to control the Reapers when interacting with them during harvests.
The Grounders are fearful and hostile towards the Reapers because they capture Grounders who are never heard from again. Grounders will avoid the Reaper tunnels at all costs, even to the point of letting Lincoln, Clarke, and Finn escape through them. A small group of Reapers were led to the Camp by Lincoln and Finn during the battle between the Grounders and the Delinquents to create a distraction for the Delinquents. Grounders will also go after Reaper raiding parties to try to rescue the people the Reapers take from them.
After the fall of Mount Weather, there is no more Red drug for the Reapers, meaning they will all eventually die from withdrawal if they do not receive medical attention.
These are the known victims of the Cerberus Project:
- Lincoln (survived and escaped)
- Numerous unnamed Grounders
Notes And Trivia
- In Greek mythology, Cerberus was the three-headed guard dog to the Underworld, making sure the dead don't escape, and the living don't go to the Underworld.
- It is unknown how the Mountain Men acquired their first group of Grounders, but a tactic similar to what they used on the Delinquents in We Are Grounders (Part 2) was likely used.
- The Reapers were the outside army of the Mountain Men. Their name is derived from their "reaping" of other Grounders to bring back to the Mountain Men for either the Cerberus or Harvest projects.
- Project Cerberus shock treatments are the real-world equivalent of Fear Conditioning. 
- Not only the Reapers crave the RED, but actually, need it to survive.
- It is likely that most if not all Reapers have died by Wanheda (Part 1) as Lincoln almost died from not having any Red in just a couple of days, and it has been three months since Blood Must Have Blood (Part 2).
- Abby was the one to discover a way to cure victims of the project in Resurrection, turning them back to normal.