Valve, the makers of ground-breaking games like Half-Life and Portal, haven't expressed much interest in publishing a third installment for Left 4 Dead, a co-operative first-person shooter series about gun-toting survivors trying to escape hordes of mutated zombies. Game designers at other studios, however, have used Left 4 Dead's gameplay as inspiration for their own multiplayer FPS games about squads fending off hordes of monsters.
Both Left 4 Dead 1 and Left 4 Dead 2 take place in a world overrun by a zombie apocalypse, the product of an airborne viral contagion that mutates people into fast-running zombies and grotesque monstrosities. The players of these games are asymptomatic survivors who must makes their way between safe-houses, scavenging weapons, ammo, and med-kits while fending off "special infected" with the ability to drag them away from the group. The big draw of the Left 4 Dead series, besides their quality blending of four-player co-op with horror, was their use of an "AI Director" to organically create scenarios of rising and falling narrative tension - fresh hordes of zombies when things were getting boring, med-kits and ammo when players needed a break.
After releasing a DLC Update called "The Sacrifice" for Left 4 Dead 2 in 2010, Valve moved on with other projects. However, Left 4 Dead 2 still gets community-designed scenarios while developers from other studios have created original "horde survival shooters" that put a unique spin on the Left 4 Dead formula.
Warhammer: End Times - Vermintide & Vermintide 2
Instead of a modern-day city consumed by a zombie apocalypse, the Vermintide video game franchise takes place in the dark and over-the-top Warhammer fantasy setting - specifically, in an empire styled after the 16th-century Renaissance that's being attacked by hordes of rat people and Chaos cultists. Instead of office workers, college students, and war veterans, the players of Vermintide and Vermintide 2 can choose from a character roster that includes witch hunters, fire mages, imperial musketeers, elven rangers, and dwarf gadgeteers, each with their own unique banter and special abilities in the style of Overwatch.
Deep Rock Galactic
Deep Rock Galactic adds several interesting twists to the formula established by Left 4 Dead, both in terms of gameplay and narrative. First, all the playable characters are Tolkien-esque fantasy dwarves working for an intergalactic mining company. Second, the objective of most Deep Rock Galactic missions is to extract mineral resources in a planet's underground caverns while fending off waves of hostile alien life forms. Third, the labyrinthine cave maps of Deep Rock Galactic are procedurally generated, creating new environments for players to navigate during each new "mining expedition."
Generation Zero
Generation Zero takes place in alternate history version of 1980s Sweden ravaged by out-of-control killer military robots. Players can choose from a character roster of teenagers from different 1980s cliques fighting to survive these marauding robots using firearms, explosives, road flares, and computer hacking. In contrast to the run-and-gun gameplay of Left 4 Dead, survival in Generation Zero is often a matter of stealth; the canny player tries to avoid being detected by patrolling robots and takes them out at a distance whenever possible.
Back 4 Blood
Back 4 Blood is currently being developed by Turtle Rock Studios, who worked with Valve to create and publish the first Left 4 Dead game. Since Valve still owns the Left 4 Dead trademark, Turtle Rock's new Back 4 Blood IP - centered around survivors trying to retake their world from hordes of zombies infested with parasitic worms - lets them create a spiritual successor to their original zombie-slaying game without running afoul of any lawsuits. The various trailers for Back 4 Blood show familiar elements of Left 4 Dead's gameplay (hordes of "Ridden" zombies, Molotov Cocktails, guns and melee weapons) while also revealing new kinds of "Mutant" zombies rival players can control to take down the co-op team, and a few "boss" monsters of truly immense size.