Stalker

"This is what happens to you if you resist.. or if you're just in the wrong place at the wrong time."

- Alyx Vance

The product of extreme, brutal Synth engineering by the Combine, a Stalker is a human who has been drastically altered both physically and mentally.

Overview

 * The Stalkers' limb extremities (hands and lower legs) have been removed and replaced with bolted-on metallic augmentations which offer reduced agility. The reasoning behind this is unknown, but would be useful in dehumanizing them further, and would cause pain with every movement.
 * Unable to talk, they will simply roar and make groans, mostly because their vocal cords have been removed (as is suggested by the augmented hole in their throats).
 * For ease of maintenance and to bring a dependence on the Combine, the Stalker's digestive tract has been removed, resulting in them surviving solely on a nutrient solution that the Combine provides, as evidenced by the gastric port in their sides. Stalker bodies are heavily atrophied, with very little muscle or fat tissue remaining.
 * All signs of their genitals have been surgically removed.

Application

 * The Stalker transformation process is performed at Nova Prospekt, after which they are transported to the Citadel, where they are required to maintain the core and guard from intruders.
 * They are transported by Razor Trains, and are confined in pods during the transportation.
 * Despite being seen in Nova Prospekt and the Citadel, Stalkers are never actually engaged in combat during Half-Life 2. However, it is possible to engage in combat with them in order to more easily achieve mission objectives in Half-Life 2: Episode One, as their lasers prevent certain tasks from being completed.
 * Episode One is also where the exact nature of Stalkers is first revealed. In Episode One, the player and Alyx enter the Citadel. After a bit of progress, the player and Alyx enter a room with two Stalkers working a Forcefield, temporarily preventing progress. Alyx states that the Stalkers "shouldn't bother us if we leave them alone". Interestingly enough, when the player is in the core of the Citadel and chooses to kill one of the two Stalkers, the remaining one will turn to face the player and roar at Freeman. The Stalker will then proceed to attack the player with the laser tool. Contrarily, if the player is fast enough to cross a field bridge there before it deactivates, Stalkers can be approached face to face, and do not attack the player but instead walk away as if he wasn't there at all.
 * It appears that the Citizens of City 17 have a full knowledge and understanding about Stalkers, what they are, and how and where they are "made." Alyx Vance comments on Nova Prospekt: "It used to be a high-security prison, it's something... much worse than that now." Barney Calhoun also makes the comment "He was about to board the express to Nova Prospekt!" at the start of the game, and sometimes Resistance members say, "They're not gonna make a Stalker out of me". However, this knowledge may just be limited to members of the Resistance.

Behind the scenes

 * The Stalker was originally planned to be fought in Half-Life 2, but this feature was cut and, later, re-introduced in Episode One.
 * The Stalkers in the Half-Life 2 Beta had melee attacks, their laser attack did more damage and they would emit a high pitched scream when killed.

Trivia

 * Stalkers can be spawned via the console in Half-Life 2, but the game will crash if they are killed, due to the lack of a death animation. In Episode One, spawning works normally.
 * Stalkers spawned with the console in Episode One are generally non-aggressive, and will usually not fight if other Stalkers are killed in front of them.
 * There is only one Stalker-related achievement, Pacifist, which can be gained by containing the Citadel Core without killing any Stalkers.
 * SMOD has a full working Stalker NPC that attacks the player and other non-Combine NPCs.

List of appearances

 * Half-Life 2
 * Half-Life 2: Raising the Bar
 * Half-Life 2: Episode One