Left 4 Dead (series)

• Crash Course is the first downloadable content (DLC) campaign for Left 4 Dead. It is free on PC, but not for Xbox 360. It was released on September 29, 2009. According to Valve it is meant to bridge the gap between No Mercy and Death Toll.
• The Passing is the first DLC crossover campaign of Left 4 Dead 2, which includes a new campaign and "new co-operative challenge modes of play" and introduces a new firearm, the M60; the Golf Club, and a new Uncommon Infected called the Fallen Survivor. It was released April 22, 2010.
• The Sacrifice is a three-chapter DLC for both Left 4 Dead and Left 4 Dead 2 and was released on October 5, 2010. Taking place after Blood Harvest and is considered to be the prologue to The Passing, as both campaigns are connected to each other.
• Cold Stream is the third DLC for Left 4 Dead 2. It was announced in a blog post on February 16, 2011. Cold Stream was released on March 22, 2011 in beta form on Steam, and it was officially released in its final form on Steam on July 24, 2012. Cold Stream was released on the Xbox Marketplace August 3, 2012 for 560MSP. It was created by map making community member Matthew Lourdelet and features various wall graffiti featuring the author's friends.
• The Last Stand is a community-made scenario for Left 4 Dead 2, and was released on September 24, 2020 with Valve's blessing as an official update to the game. The update includes over twenty new maps for survival mode, a new campaign, and additional updates to the game.

• The Mercenaries: No Mercy makes a return on the PC version of Resident Evil 6. As an exclusive to No Mercy, Capcom and game developer Valve had teamed up to allow Left 4 Dead 2 content for No Mercy. The content includes the main cast from Left 4 Dead 2 being playable characters with their own loadouts alongside the Witch and the Tank replacing the Bloodshot and the Napad as enemies.
• Pixel Force: Left 4 Dead is a downloadable indie game where the game is played as if it were released for NES.
• Left 4 Dead: Survivors (Left 4 Dead: 生存者たち, (Seizonshatachi)) is a version of Left 4 Dead 2 released on December 10, 2014 for Japanese arcades by Taito.[1] The game features the same scenarios and locations as L4D2, but features a different cast of survivors unique to this edition.
• The No Mercy level of Left 4 Dead appeared as DLC for the cooperative-shooter Payday: The Heist and its sequel Payday 2.[2]
• On August 20, 2015, a free update was released for the PC version of the game Zombie Army Trilogy which imported the eight survivor characters from the Left 4 Dead games.[3]
• In March 2017, asymmetric horror game Dead by Daylight released the "Left Behind" DLC for PC, which unlocked Bill as a playable character. It also unlocked Zoey, Ellis, Francis and Rochelle costumes for Meg, Dwight, Jake and Claudette respectively.[4]

Tied to the release of "The Sacrifice" downloadable content, a four-part web comic drawn by Michael Avon Oeming was released to describe the events of the first set of survivors, Bill, Louis, Zoey, and Francis, that led them to meet the second group, Coach, Ellis, Rochelle, and Nick.[5][6] Each of the parts provides a backstory for each of the first survivor characters at the onset of the pandemic, and also follows directly up on the final campaign from the first game, "Blood Harvest", where the survivors, identified as carriers, are taken to a secure military facility. The use of the comic medium to expand the game's story outside of the game's bounds has been used by Valve before in promoting Team Fortress 2.[7]

Within Left 4 Dead 2, a fictional southern rock band called the Midnight Riders was introduced. Several songs have been produced for the game and tie-in promotions. Two of them, "Midnight Ride" and "One Bad Man", have appeared as downloadable content for the Rock Band series via the Rock Band Network.[8] Valve released a series of holiday cards themed after the survivors, created by Alexandria Neonakis.[9] Three plush toys have been released based on the Left 4 Dead series' infected; first with the Boomer, and later with the Hunter and the Tank. They are to be followed by two more plush toys yet to be released. They were also created by Alexandria Neonakis.[10][11]

In June 2011, it was confirmed that Valve had signed a deal with NECA, to produce action figures, which includes Left 4 Dead and other Valve series. There are two figures produced so far, Left 4 Dead's Boomer (which was released in June), and Left 4 Dead 2's Smoker (which was released in November).[12]

Valve has not yet made any direct statement related to future games in the series, though Chet Faliszek said in 2012 that like Valve's other games, a sequel had not been ruled out.[13] Rumors from 2013 through 2016 led players to believe that Valve was developing Left 4 Dead 3, with a target release date in 2017.[14][15][16][17] However, by this point, Turtle Rock was no longer working with Valve but instead had been developing the asymmetrical multiplayer game Evolve.

In April 2019, Tyler McVicker of Valve News Network reported that while Left 4 Dead 3 had been in development at some point at Valve, it had been subsequently cancelled a few years earlier. Assets that McVicker had obtained showed a level comparable to assets used in Counter-Strike: Global Offensive, and indicated that very little other work appeared to have been done on the title.[18]

In July 2020, multimedia storybook The Final Hours of Half-Life: Alyx, which focused on the 2010s at Valve, gave information about the cancelled Left 4 Dead 3 project that was developed around 2013, describing it as an open-world game set in Morocco where the player fights hundreds of zombies. The project was shelved due to the slow development of the Source 2 engine.[19][20]

At The Game Awards 2020, Turtle Rock announced Back 4 Blood, their spiritual successor to Left 4 Dead 2, slated for a June 2021 release date.[21]

The Left 4 Dead series uses a collection of artificial intelligence routines, collectively the "AI Director", to monitor and alter the gameplay experience in response to the players. Valve's primary goal with the AI Director was to promote replayability of the games' campaigns, as their previous multiplayer games with this feature, such as Counter-Strike and Team Fortress 2 have shown thriving communities of players that continue to play the games despite the limited number of maps available due to the unpredictable nature of online play.[22] In considering this for Left 4 Dead, Valve identified that many games use static events that always occur at fixed points in a level, or limited dynamic events where one of several events could occur at fixed points. Valve themselves had used this idea in some key battles in Half-Life 2: Episode Two where the spawning of Combine forces would be based on the player's location.[23] They recognized that such systems do not promote replayability or cooperation: players could easily memorize where events would occur, and those that had yet to experience the events would slow other players down.[22] With the concept of the AI Director, Valve believed it could capture the same chaos and randomness that would occur in Counter-Strike and Team Fortress 2 in the cooperative gameplay experience, transforming it from simple memorization to a skills challenge.[22] Valve has further termed this approach "procedural narrative", creating a new story each time the game is played.[24]

The AI Director has several facets that combine to form "structured unpredictability" for each playthrough.[22] The Director first procedurally generates a dramatic flow for the level, which identifies the size and location of hordes of common infected and uncommon infected throughout the level. The procedural generation considers each traversable area on the map, using pathfinding algorithms that Valve had incorporated into Counter-strike computer-controlled characters, and the "flow" of the map—the general direction from the start of the level to the safe house. As players progress through the map, the Director will spawn infected in areas near the players and out of sight, while removing infected from earlier portions of the map the players have passed through.[22] The placement also considered the "Escape Route", the shortest path through the map, and will increase encounters along this route to increase the difficulty.[22] Spawning rules are different for each of the infected; hordes are more often spawned behind players to take them by surprise, while special infected like the Hunter and Smoker will spawn ahead of the players, giving their individual AI the opportunity to lay in wait for the players.[22] Procedural generation is also used to place weapons and other items throughout the level.[22] Weapons and equipment can be programmed by the map designer to be generated by the Director at fixed points, allowing for some predictability for the players, and to give some creative control to the design for story-telling and visual effects.[22]

Overriding the procedural generation is the aim to create "active dynamic pacing" of the game by continuous monitoring of players, and altering the predetermined pacing to react to this.[22] Each player character is tracked by a metric called the Survivor Intensity, which increases as the character takes damage, becomes incapacitated, or another character dies nearby, among other effects, and slowly decays in time. The Director will alter its previously developed schedule for spawning of infected to build up Survivor Intensity to a certain threshold; when this occurs, the game sustains this peak for a few seconds, then enters a period where it relaxes and reduces the spawning of infected, allowing the players to finish their current encounter and allow their Survivor Intensities to fall away from the threshold. The Director then repeats this cycle until the players have reached the end of the level.[22]

The boss infected encounters are generated through different means, creating a sequence based on cycling without repetition between three situations: Tank, Witch, or "No event".[22] Boss events are then generated based on the number of areas the players have passed through. These events are not modified by the dynamic pacing of the game; Valve discovered when these were controlled by the pacing, players would often be at too high of an intensity to allow these encounters to occur. Keeping the events on a separate track allows the pacing to be unpredictable, further enhancing the replayability goal of the Director.[22]

A sample series of encounters (with progress from left to right) during a level playthrough of Left 4 Dead.[22] The top bar shows the original population of common (white icons), special (colored icons) and boss (large icons) procedurally generated by the AI Director prior to starting the level. The second bar is the monitored "Survivor Intensity", with the red "Relax" sections occurring immediately after the Intensity reaches a threshold limit. The third bar is the actual population generated while playing the level; certain common hordes have been removed due to the Intensity being high and in a "relaxed" state, while other hordes and uncommons have been added when the Intensity is very low near the end of the level.

The Director also creates mood and tension with emotional cues, such as visual effects, audio cues for specific types of infected, and character communication.[25] Within Left 4 Dead 2, the Director has the ability to alter placement of walls, level layout, lighting, and weather conditions, and reward players for taking more difficult routes with more useful weapons and items.[26][27] A dedicated Director routine dynamically controls music and other ambient sound effects, monitoring what a player has experienced to create an appropriate sound mix. The process is client-side and done by a multi-track system. Each player hears their own mix, which is being generated as they play through the game, and dead players watching a teammate hear their teammates' mix.[28]

Valve sees the AI Director as a tool to be used for other games. A software patch for the cooperative game Alien Swarm added a game mode, Onslaught, that used a version of the AI Director from Left 4 Dead to dynamically generate enemies for the players to fight.[29]

Valve has investigated the use of biofeedback from the players directly as input for the Director. At the 2011 Game Developers Conference, Valve demonstrated a simple test where a player, fitted with a device to measure skin conductance level, played through a Left 4 Dead 2 level. Certain in-game actions, such as hearing a threatening noise or passing an opening doorway, would cause the skin conductance to rise. Valve postulates that such information fed into the Director could create a much more effective player experience.[30]

The Infected characters within the game are divided into four categories. The most abundant are the "common" infected, the least deformed of the infected creatures. Alone, these infected are weak, but in large numbers, they pose a significant threat to the Survivors. Often, the Director will engineer a "horde", where a large number of the common infected charge the players. Hordes can also be created by setting off in-game "crescendo events" that usually involve some loud noise created by the players, or by activating a car alarm. As such, many individual levels feature a unique, noisy event, such as the raising of a rusty elevator, where a very large horde will be attracted, with the players given forewarning so that they can prepare for the onslaught.

A second type of infected introduced in Left 4 Dead 2, are the "uncommon infected". Uncommon infected are unique to certain campaigns and, though not any stronger than the common infected, have a special ability that can hamper the players' progress. For example, the uncommon infected in "Hard Rain" are infected road workers that are still wearing protective ear pieces, making them immune to the lure of pipe bombs.

The third type of infected creature are the "special" infected. Each of these creatures appear less frequently during a campaign, and are designed to keep the players working together as a team. They have very specific vocalizations, as well as leitmotifs which are inserted into the game's soundtrack when the AI Director places one, allowing Survivor players some forewarning. They serve as playable characters for the Infected in the games' versus modes.

• Hunter - a nimble and agile Infected that is able to leap large distances and tackle a single Survivor, who will take damage until either they are killed or the Hunter is either killed or shoved off. The pounced Survivor is rendered completely incapable of taking action, encapsulating the intent of the special infected: they must be rescued by other Survivors as they cannot help themselves.
• Smoker - an Infected with a long tongue that it uses to ensnare and strangle Survivors, rendering them helpless until the tongue is hit or the Smoker killed or stunned with explosives or shoving. When killed, it leaves a cloud of smoke that temporarily obscures the Survivors' vision and makes them cough.
• Boomer - a slow-moving bloated Infected that vomits bile onto Survivors; though not harmful itself, the bile attracts common infected to the character. When killed, a Boomer explodes, covering any nearby Survivors in more bile.
• Spitter - an Infected that spews a large amount of acidic "spit" at the Survivors. Survivors standing in the "goo" suffer quickly-increasing damage until they move out of the puddle. The Spitter herself is quite weak and upon her death, she leaves a small puddle of acid that also damages Survivors.
• Charger - an Infected with a large mutated arm that attacks by charging Survivors at high speed. The charge attack is capable of knocking down and effectively stunning every Survivor at once, and the Survivor at the forefront of the attack is carried as far as the Charger runs or until the Charger hits a wall, at which point the Charger begins to pummel the Survivor. Survivors being pummeled are completely helpless and will be killed unless the Charger is killed. The Charger is also unique among the Special Infected in that, due to its hulking size, it cannot be shoved; only frag rounds and explosives can stumble a charger. The charger can cause almost as much damage with its standard melee attack while not charging.
• Jockey - a small Infected with a hunchback that can "ride" a Survivor, hence the name. A Jockey can control a Survivor's movement while riding them, steering the Survivor into danger. A Survivor suffers damage as long as a Jockey rides them and is completely helpless unless the Jockey is shoved or killed.

Finally, there are two special "boss" infected characters that the AI Director includes at rare moments:

• The Witch - an infected woman with glowing eyes and long clawed fingers. Witches are unique in that they are passive, preferring to be left alone to cry. However, if a Survivor "startles" her, either with their flashlight, by invading her space, or by damaging her, she will charge that Survivor and automatically incapacitate them. Once incapacitated, the Witch will viciously attack the downed Survivor until either they are killed or she is. If she kills the Survivor who initially disturbed her, then she will run away. On higher difficulty levels, a Witch can automatically kill a Survivor. She is not particularly durable, but she can run faster than Survivors, making it difficult to slay her before someone takes damage. She is the strongest and fastest infected on foot. However, she has the second most hit points, behind the Tank. She can be stunned by explosives and frag rounds.
• The Tank - an enormous Infected with superhuman strength. The Tank can charge Survivors and smash them to the floor, or can throw debris, rocks, and cars at them, the latter resulting in instant incapacitation. Kiting is a valid tactic, but Survivors must do so for an extended period of time, a Tank on Normal difficulty has ten times the health of all the Survivors combined. In versus —modes, players may not control the Witch, and may only play as a Tank if the AI Director has already chosen to spawn one. He is the second strongest infected and has the highest number of hit points by far over all of the other infected. Due to his gargantuan size, the Tank can only be stumbled by exploding oxygen canisters or propane tanks. In Left 4 Dead: The Sacrifice, he is Bill's murderer.