Left 4 Dead is a series of cooperative first-person shooter survival horror video games created by Turtle Rock Studios and published by Valve.
Set in the days after a pandemic outbreak of a viral strain transforming people into zombie-like feral creatures, the games follow the adventures of four survivors attempting to reach safe houses and military rescue while fending off the attacking hordes.
Players use a combination of melee weapons, firearms, and thrown objects to fend off attacks from the bulk of the infected creatures, while using an assortment of healing items to keep their group alive.
[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.
[19][20] At The Game Awards 2020, Turtle Rock announced Back 4 Blood, their spiritual successor to Left 4 Dead 2, released on 12 October 2021.
Within the United States, the Civil Emergency and Defense Agency (CEDA) orders the creation of safe zones with the aid of the military, and evacuates as many people as possible to these areas, aiming to transfer them to islands and ocean-going ships, as the infected are unable to cross bodies of water.
In Left 4 Dead 2, limited-use weapons such as the chainsaw, grenade launcher or the M60 machine gun can also be carried in a similar manner.
Should this bar drop to zero, the character dies, and can only be restored either through a defibrillator, appearing later in a level in a "rescue closet" from which they must be freed, or when the remaining players reach the safe house.
For all but the last chapter, the goal of the players is to reach the safehouse at the end of the level, where fresh supplies of weapons, ammo, and health items are typically found.
Other modes are based on single-situation standoffs where the Survivors have to hold out as long as possible, or where two teams compete to fill a generator with as many gas cans as possible.
For example, one mutation may give all the Survivor characters chainsaws from the start, while another may make every unique infected appear as a specific type, such as the Tank.
Valve has further supported the user community by highlighting popular third-party maps, and including select ones in software patches for the game.
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.
[21] 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.
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.
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.
[21] 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".
[24] 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.
Certain in-game actions, such as hearing a threatening noise or passing an opening doorway, would cause the skin conductance to rise.
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.
Special Infected 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.
Finally, there are two special "boss" Infected characters that the AI Director includes at rare moments: The Left 4 Dead series has a considerably large and active fanbase; a large amount of community-made content are available for the two games, with The Last Stand, a content expansion in particular being blessed by Valve and released as an official update for Left 4 Dead 2.
[33][34] Said lore is highly absurd and nonsensical in nature, such as his birth in a candle factory and turning purple through an iron-rich diet, which made him "incredibly attractive to women", and that he has a daughter who had an affair with Ronald Reagan.
[36] While the edits were eventually reverted a few days after creation, the Left 4 Dead fan community seemed to embrace the concept of Purple Francis, and a mod on Steam Workshop was released on February 14 by a developer who had worked on The Last Stand update, claiming to restore the "cut content" depicting Purple Francis.