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.
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.
When Miles Upshur tries to unlock the Administration Block doors to escape, he is ambushed by Father Martin, who sedates him with a syringe. Before Miles passes out, Father Martin shows him security footage from the Underground Lab, which depicts a team of Murkoff PMCs being ambushed and slaughtered by the invisible entity upon entering the lab. While Miles is traversing through the prison showers, its silhouette can be seen at the bottom of the stairs before vanishing. Shortly afterward, while walking through a Sewer tunnel, Miles catches a glimpse of the entity as it disappears through a barricade. Soon afterward, two men are heard running from the Walrider, before being caught by it and torn apart. Miles arrives at the scene minutes later, discovering their dismembered remains on the floor. In the Courtyard, it is seen scouring the air, as well as one brief 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 managed to escape, slaughtering most of the facility's staff members, which directly led to a massive prison breakout.
When Miles reaches the underground facilities, he discovers the bodies of the men he saw being murdered on the security cameras. Miles tries to escape through an open hangar, but the Walrider assembles in front of it, causing the security systems to shut down all exit routes, forcing Upshur to retreat the way he came.
As the Walrider gives chase, Miles runs back to the exit, only to be greeted with Chris Walker at the lab's entrance, who tackles the journalist to the floor. Unknowingly having put himself in the path of the incoming swarm, Walker mocks Upshur one last time, but before he can kill his intended target, Chris is grabbed by the Walrider from behind and spiraled away from Miles. The Walrider relentlessly throws Walker against nearby walls, and eventually pulls him through a ventilation shaft, dismembering his body in the process.
Before he can recover from his ordeal, Miles is suddenly guided by an elderly voice into a nearby secured office. The voice reveals himself to be Rudolf Wernicke, a presumed deceased doctor who explains how William Hope gained control over the Walrider and how he's also responsible for keeping Wernicke alive out of love, even though Rudolf wants to die more than anything. He instructs Miles to disable the power grid for Billy's life-support pod to undo what he has made and murder Hope. 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 gunmen. The mercenaries open fire, seemingly killing Upshur, but Wernicke realizes that Miles has become the sentient host. As the screen fades to black, the Walrider can be heard emerging and attacking the soldiers as they shoot and scream in agony.
The Walrider first appears in the Hospital after Waylon is freed from his restraints, murdering two patients on either side of his glass cell. Upon escaping, Waylon walks down the plastic tarp hallways of the hospital, the Walrider appears and chases him into a room where several patients are stabbing a doctor's corpse on an operating table.
While crawling through a vent, Waylon overhears a guard and a doctor discussing ways of dealing with the situation. After the two leave, Waylon drops down and catches a glimpse of the Walrider's silhouette just outside the doorway. After opening a decontamination gate from security control, Waylon finds himself pursued by the Walrider but escapes through the decontamination chamber.
In the morgue, Waylon climbs through an opening near the exhaust chimney as he spots the Walrider levitating through the shaft before disappearing.
In Prison, Waylon spots a silhouette of the Walrider chasing two inmates down a hallway. The group can be heard being killed by the entity as they scream in agony.
In Drying Ground, Waylon spots the Walrider in one of the buildings leading up to the water tower. The swarm completely ignores Park and instead traverses through the grated floor below.
After Waylon escapes from Vocational Block, he finds himself in the Male Ward, now swarming with tactical officers instructed to shoot and kill on sight. Avoiding the mercenaries, Waylon overhears two of them requesting immediate backup in the underground facilities as the Walrider, an entity unknown to the PMCs, slaughters their teammates.
Upon reaching the Exit, Park discovers an injured Jeremy Blaire as he's resting on the main doorway. Waylon ignores Jeremy's plead for help and attempts to leave, before Blaire ambushes and stabs him with a hidden knife. As Blaire is about to deliver the killing blow, the Walrider emerges and throws Jeremy into the air and across the room, before entering his body and dismembering him apart from the inside.
Wounded and exhausted, Waylon hobbles his way through the asylum's gates and enters Miles' abandoned jeep, before noticing through the windshield a dark cloud hovering in front of the asylum. Waylon then picks up the camcorder to look more closely, noticing Miles standing in the midst of it, limping towards the exit. Before having to think twice, he struggles with the gear and attempts to turn the vehicle around. As the car gets surrounded by the nano swarm, Waylon accelerates and rams through the asylum's main gates.
Outlast: The Murkoff Account
In Issue 3, it's revealed that, despite being apparently killed by Miles, the Walrider, possessing Billy's body, later turns up at the caravan of Billy's mother, Tiffany Hope. The body of Billy was then, after Tiffany was slaughtered by the Walrider, blown up in an explosion, presumably ending Billy's life for good. What Murkoff doesn't know is that the Walrider wasn't destroyed, but merely switched hosts for a colony of ants.
- 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 and Eddie Gluskin, such as pulling and throwing the player out of a confined space, while they're either crouched or passing between a gap.
- In the game's configuration, the Walrider is dubbed as "NanoCloud".