Development and ReleaseEditA number of influences for the game have been acknowledged. In addition to the Slender Man character, the movement rules for Slender Man resemble those for the "unintentionally creepy" movement of the player's companion Watson in Sherlock Holmes: Nemesis, where a similar algorithm was used to enable Watson (a friendly character) to appear to be following the player without needing to implement pathfinding or walking animation for him. The gameplay and some aspects of the artistic style of the game are also extremely similar to a prototype game named "Hide" that was posted on the Super Friendship Club gaming forum in 2011, although the movement of the enemies in Hide is different and the player can view them from a distance without harm. Many of the game's graphics and code snippets are obtained from Unity's own Asset Store, the defaults included with Unity, or from contributors on the Unity Forum thread for the game.
After the game's release, several other games were released that were based on Slender: The Eight Pages, usually based on exploration mechanics with similar AI for the enemy. The most popular games based on Slender: The Eight Pages are Slenderman's Shadow which was released in several maps and "Slender-Man" for iOS, which reached number two in worldwide app ratings, being outsold only by Bad Piggies. Several mods were also released. A more advanced version of the game implemented using the Source engine was originally announced named Slender: Source, but this attracted criticism as Hadley was not involved in the development of the game. In response to this, Slender: Source was renamed Faceless.
In Version 0.9.4 there is an unlockable "Daytime Mode". To unlock this mode, the player must beat the game as normal, but now there is a new ending where the player awakes in the daytime after collecting all eight pages and is caught by Slender Man. After waking up, the player can walk around in the daytime for a few seconds before the credits roll. "Daytime Mode" is simply during the day and without a flashlight. If the player beats "Daytime Mode" the ending will be the same as before, only the player wakes up at night. After the credits roll, the player will have unlocked "$20 Mode". In this mode if the player sees Slender Man a song called 20 Dollars by American rap artist Ron Browz starts playing. This is a reference to a fan originated meme about Slender Man. This is the only version in which the player selects these modes in the "Options" portion of the menu screen, and this is also the only version in which two hidden modes can be played at once ("Daytime Mode" and "$20 Mode").
From version 0.9.5 on successfully completing the game for the first time in an earlier version unlocks "MH Mode". This mode starts off and ends like an "Entry" from the web series Marble Hornets, which is based on the Slender Man mythos. This mode also suggests that the round of gameplay is actually a prerecorded video. There is static on the top and bottom of the screen, and the different types of music that play as the player collects more and more pages do not play. If the player beats the game for the first time in this version (without having beaten it in an earlier version first) they not only unlock "MH Mode", but unlock "Daytime Mode" aswell. Beating both of these modes will then unlock "$20 Mode". These modes are, in this version and on, listed in the "Extras" portion of the menu screen.
From version 0.9.7 on there is ambience during the menu screen, as well as portions of the eight pages being shown. There are now links to the Slender Man mythos forums and some popular web series based on the mythos such as Marble Hornets. As of this version, the game was officially renamed Slender: The Eight Pages. "$20 Mode" has been removed due to the copyrighted material. "MH Mode" has been renamed to "Marble Hornets Mode" (simply the unabbreviated version of the title). Two new light sources are available: a glowstick with infinite light but a very short range, and a crank lantern with a wide range but short amount of light which must be replenished by cranking the lantern. The player can now pause the game by pressing escape, but only when there is no static on the screen as to not allow the player to leave in response to being frightened.
The game received overwhelmingly positive reviews. Some publications noted the effectiveness of the game's minimalist horror approach, contrasting it with many action-based survival horror games of the time period, such as the latest Resident Evil and Silent Hill titles. Gaming website IGN called Slender "pure horror", claiming "few horror games thrust you directly into the heart of fear". Explosion scored the game a 4/5 and said that "Slender is the kind of game that unearths that long-lost sensation inside of you whereby you’d play a game for the addictive novelty value it possessed and not the budget that was thrown at it."
The official website crashed after a flood of people tried to download the game, leading to subsequent mirror downloads on Reddit and later MediaFire.
- Main article: Slender: The Arrival
A sequel to Slender: The Eight Pages, entitled Slender: The Arrival, is currently in development by ParsecProductions and Blue Isle Studios. Little is known about what the game will include, but based on information released on the website, players can expect "more levels, improved visuals, and an engaging storyline." IGN has described it as "a simple, devastatingly effective game of harrowing escape. It is unknown when the game will be released. Unlike the original Slender: The Eight Pages, the game will not be free.