The Mount Massive Asylum Incident was a massacre of the staff of Mount Massive Asylum that took place a few hours prior to the start of the original story, and is witnessed firsthand by Waylon Park, resulting in the establishment being overrun by the Variants.
In seeking to create a perfect Walrider, Murkoff Corporation used the Morphogenic Engine to conduct a series of experiments on the asylum's patients through dream therapy, which caused a number of physical deformities and severe psychological trauma—another effect it had was to make them stronger. Billy Hope, one of the patients, managed to control the Walrider by self-directing the lucid dream states, allowing him to wreak havoc on the facility.
Once Billy took control of the Walrider, he used the entity to slaughter Murkoff personnel and patients within the facility. In the ensuing chaos, many Variants escaped and they too began wreaking homicidal havoc in their extremely unstable states of mind. The security forces desperately tried to quell the attacks and stop the patients, but ultimately failed, losing the facility to the inmates as the emergency evacuation was put into action.
Following their initial failure, Murkoff dispatched a group of private military contractors who arrived in the midst of chaos with several MRAP trucks and riot gear. Despite being well-armed, the teams were soon slaughtered by the Variants and the Walrider.
Many of the escaped patients started to freely roam the asylum with no set purpose. Some of them took on a sort of religion and started to worship The Walrider, thinking that it was a god. These variants became followers of Father Martin, who was attempting to release The Walrider into the world. Chris Walker, another escaped Variant, was trying to prevent this from happening. Many Variants enacted their murderous revenge against the Murkoff personnel, whilst other patients participated within the chaos. A number of inmates committed atrocities such as cannibalism, necrophilia, rape, genital mutilation and physical torture.
In the underground compound, Miles Upshur, an independent journalist who arrived at the asylum based on an anonymous tip, terminates Billy's life support and seemingly kills him. The Walrider, now without a host, attacks Miles and fuses itself into his body. A severely wounded Miles limped toward the lab's exit, only to be greeted with Rudolf Wernicke and several Murkoff tactical officers who gun him down. As Wernicke realizes that Miles has become the new host, the Walrider emerges and slaughters the troops as they scream in agony.
Elsewhere, Waylon Park cuts through the asylum's Male Ward in search for an exit as he overhears the PMC reinforcements being permitted to shoot on sight as they make their way to the basement lab after being radioed for help.
Shortly after, Park hobbles through the asylum's front doors and heads towards Miles' jeep after being ambushed and wounded by Jeremy Blaire, his superior who attempts to silence him before meeting his end at the hands of the Walrider. As he prepares the vehicle, Waylon notices through the windshield a dark cloud hovering in front of the hospital's entrance. Zooming in with the camcorder, he notices Miles standing in the midst of it, limping toward the car. Before having to think twice, he struggles with the gear and attempts to turn the vehicle around. As the jeep gets surrounded by the nano swarm, Waylon accelerates and drives through the asylum's main gates.
The asylum was later secured by Murkoff's Tactical Division and cleaned up. Once recovered from his injuries, Waylon comes into contact with a man associated with a leaking website. The person informs Park that his footage will be more than enough evidence to ruin the Murkoff Corporation, but warns Park that doing so will result in the company doing everything it can to punish him in return, including threatening his family. Despite some initial hesitation, Park ultimately decides to take the risk and uploads the video.
Sometime after, Waylon burns down his house and goes into hiding with his family. When their search for Park ends up going nowhere, Murkoff, in retaliation, sends their mitigation agents to hack his online account and attempt to discredit him by publishing outlandish and poorly written articles.