The Walrider, also known as The Swarm, is the main antagonist and the final enemy encountered in Outlast and Outlast: Whistleblower. It is the source of the madness that seems to infect most of the asylum's inhabitants and the deity of Father Martin's religion and his followers. It is first fully seen at the start of the Sewer chapter, after escaping from the Prison Block, where it can briefly be seen towards the end of the chapter. The Walrider is also encountered early on in the DLC.
While appearing ghostly in nature, the Walrider is actually a swarm of nanites; small, nanoscopic machines which collectively possess great strength and power. It took possession of William "Billy" Hope. As it is shown by a document, 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 faint black aura, unless it comes into very close proximity to the player. Miles can only see it clearly through the night vision on his camcorder. Despite having no actual bones, muscles or internal support structure to speak of, the Walrider's nanites are capable of immense physical strength, and it 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. The Walrider is relentless in its pursuit, while doors will slow its progress somewhat, it is capable of going underneath them. It is extremely difficult, if not impossible, for the Walrider to lose track of the player in a chase scene, only stopping once the player passes through a decontamination gate, which the Walrider cannot pass through.
The Walrider is seen multiple times throughout the game, its 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 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 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 realizes 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 areas, it is simply there for visual effects and will not harm him. At the break of dawn, it is heard slaughtering the tactical team who killed Miles in the original game. Near the asylum's entrance, the Walrider kills Jeremy Blaire in a manner similar to how Chris Walker was slaughtered. At the end, the form of Miles inside of the swarm of nanites is visible from Miles' car, and moves towards Waylon. While starting the car and turning towards the gates, the Walrider dissipates around the car, before Waylon accelerates and rams through the front gates.
- Main article: The Walrider/Gallery
- The Walrider is based on the nightmare creature from German folklore called "Alp", who is also called the "Walrider". Alp is said to originate from the mountains of Germany, similar to how the Walrider originated from Mount Massive. Both of them also have the ability to fly and turn partially invisible, as well as shapeshift.
- Since the Walrider relentlessly pursuits the player, the only way to successfully hide from it is either by crouching behind a soda vending machine or in a bathroom stall. From there, the player can spot it roaming the area via night vision. It's possible to sneak away in this situation, however, the Walrider will still spawn during the forthcoming scripted events.
- The Walrider shares some animations with Chris Walker, such as pulling and throwing the player out of a confined space, while they're either crouched or passing between a gap. This can also be seen with Eddie Gluskin.
- In the game's configuration, the Walrider is dubbed as "NanoCloud".