The Walrider also known as The Swarm is the main antagonist and the final enemy encountered in Outlast. It is behind the madness that seems to infect most of the asylum's inhabitants. It is the deity of Father Martin's religion.
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 is shown by this note, 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 it's progress somewhat, it is capable of going underneath them.
The Walrider is seen multiple times throughout the game, it's presence can be seen in the Courtyard scouring 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 making progress to be the perfect host, with his self-directed lucid dream states, however the situation got out of control. The Walrider, controlled by Billy, slaughtered everyone except one. Dr. Wernicke was kept alive by Billy all this time. When Miles reaches the Underground 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.