Zombie Panic! Source

Zombie Panic! Source is currently available on Steam.
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Supported OS:

Windows •

Linux +

SteamOS (See system requirements)

Zombie Panic! Source (ZPS) is a cooperative, survival-horror Half-Life 2 modification. It is the sequel to the popular Half-Life modification Zombie Panic. Set in the middle of a zombie apocalypse, players start as a small group of survivors attempting to stay alive. Each map will have its own objectives to complete and win the round or survive by a period of time.

Initially released on December 28, 2007 via ModDB page page as a public beta (v1.0) and later re-released for Steam via Steamworks in October 8, 2008 as a free-to-play game.

It comes with it's own SDK, named Zombie Panic! Source Mod Tools.

Gameplay

At the start of each round, players can either choose to join the human team or volunteer to be the first zombie. If no one volunteers the game will pick one human randomly, and the game begins! The starter zombie's goal is to kill the humans while the human goal is to stay alive as long as possible, complete objectives, or even wipe out all the zombies. The catch is that when a human dies he will simply join the ranks of the undead, now ready to finish off his old living teammates. The humans can't tell by the player list who is alive and who is undead.

Eventually, there will only be one survivor still standing if everything goes wrong, his back against the wall while facing the animated corpses of his former allies. In a survival round, the zombies team only has a certain number of reinforcements (lives), and when a zombie is killed they lose a life from the counter. However, when a human is killed they gain 1 life. When the zombies run out of lives, the remaining survivors win the round by killing off the remaining undead left on the map. In objective rounds, zombies get an unlimited number of lives while the humans must complete a variety of map-based objectives to win.

Features

Human abilities

Speed - A zombie can only run at about 80% of the top speed of a human player. However, the more weaponry and ammo a human picks up, the more it will slow him down, making him an easy target for the undead. If they are carrying nothing, a human can easily outrun a zombie. Watch out so you don't get cornered...
Weapons - The humans have a wide selection of weapons to use for defending themselves, like firearms, explosives, and melee weapons. It is essential for humans to locate weapons and ammo around the map in order to survive since they only spawn with a random melee weapon and handgun scavenging is a must. Remember that inventory space is limited and hogging weapons will merely slow a survivor down, so sharing with teammates is a must!
Numbers - At the beginning of the round, there are only 1-3 zombies, depending on the server is population. Everyone else is a survivor. This means they can easily watch each other's backs and provide cover fire. However, once their ammo starts to run dry and they begin to squabble amongst themselves for the few precious ammo clips available...
Communication - Instead of typing messages to each other (occupying busy hands), which can be really dangerous when the zombies are lurking in the shadows, the humans have the ability to communicate by voice commands. There is a menu with several commonly used commands to help the survivors organize a defense, trade ammunition, and request help.
Panic! - Drop everything but what is in your hand and flee! This grants a temporary speed boost, allowing for a quick getaway, but use it wisely. More often than not, panicking just staves off a survivor's inevitable doom.

Zombie abilities

Regeneration - Over time, zombies regenerate hit points, making it more difficult for survivors to make the undead dead.
Carrier - The first Zombie of the round differs a lot from the other undead - he looks different, sounds different, and has abilities that the average zombie can only dream of. He is the Carrier! With the power to infect people (10% infection chance per hit by default) he will cause panic and paranoia amongst the living. He also has the ability to roar, summoning other zombies to his aid.
Persistence - Zombies might seem to die often, but they won't give up. They just keep re-spawning over and over until your pistol runs out of bullets, your back is to the wall, and there is no escape.
Numbers - Later on in the game, as the undead ranks swell with fresh dead, the last few humans will find their superior weaponry of small comfort for the hordes of zombies closing in.
Zombie Vision - Zombies can't use flashlights, so they have the ability to use Zombie Vision. Z-vision works in a way that it brightens up any possible light source in a room to make it more viewable. More importantly, when humans get tired, they start to glow blood red, making it easier for a zombie to track their prey. To prevent games of hide and seek a survivor who just sits still will also start to glow red through sheer terror of the unknown.