The Walrider also known as The Swarm is the main antagonist and the final enemy encountered in Outlast and the DLC Outlast: Whistleblower. It is behind the madness that seems to infect most of the asylum's inhabitants and is the deity of Father Martin's religion and his followers. It is first encountered at the start of the Sewer level, after escaping from the prison. The Walrider is also encountered early on in the DLC.
While looking ghostly in nature, the Walrider is actually a swarm of nanites; small, nanoscopic machines who possess great strength and power. It took possession of William "Billy" Hope. As it is shown by thisdocument, when Billy was undergoing Morphogenic Engine Therapy, he learned to self-direct the lucid dream states, he was then capable of controlling the Walrider.
As it is composed of nanites, it is nearly invisible to human eyes, only showing a feint black aura. Miles can only see it clearly through the night vision on his camcorder. The Walrider has immense physical strength, and is the most physically powerful entity in the game. It can easily overpower any Variant, including the large, muscle-bound Chris Walker, as is seen in the Underground Lab when Walker is brutally murdered. When attacked by the Walrider, the result will be an instant death. The Walrider is relentless in its pursuit, while doors will slow its progress somewhat, it is capable of going underneath them.
The Walrider is seen multiple times throughout the game, its presence can be seen in the Courtyardscouring the air, as well as one encounter behind the locked gate. Project Walrider was the process of exposing patients to Morphogenic Engine Therapy to create a suitable vessel for the nanite swarm, for only a person that has seen enough horror can become the host. The aim was to create the perfect host for the Walrider, so that it could become a sentient being, as is evidenced by these notes: Gods and Monsters,Variant Postmortem. It is likely that The Variants are the results of this procedure. Billy was capable of controlling the Walrider, with his self-directed lucid dream states, however because they were only using damaged and unstable minds to experiment on, the situation got out of control. The Walrider slaughtered everyone except one. Dr. Wernicke was kept alive by Billy all this time. When Miles reaches theUnderground Lab, Dr. Wernicke, who was thought to be dead, instructs Miles to disable the power of Billy's life-support pod to undo what he has made, to murder Billy. After Miles shuts down the life-support pod, the Walrider, now without a host, attacks Miles and fuses itself with his body, taking him as its new host. As Miles limps towards the exit in pain, the door opens, revealing Dr. Wernicke and several soldiers. The soldiers open fire, but Wernicke realises that Miles has become the sentient host. As the screen fades to black, the Walrider can be heard attacking as the soldiers scream in agony.
In Outlast: Whistleblower, the Walrider makes several appearances. In the Hospital, it will attack Waylon on sight in some areas. However, throughout most of the DLC, it is simply there for a visual effect and will not harm him. At the end of the DLC, after taking Miles as its host in the first game, the Walrider kills Jeremy Blaire in a manner similar to how Chris Walker was killed. At the end, Waylon exits the asylum and takes Miles' car to leave but before he can turn it on, he grabs his camera one last time and zooms at the entrance to see Miles surrounded by the nanites, walking towards him. Waylon hurries to turn on the car before the Swarm can catch him, and drives through the locked gates to blast them open and leave.