We first announced this story way back in February, and at long last it has arrived. Let it be written that on April 21, 2009, the Left 4 Dead Survival Pack was released. If you have been waiting eagerly for this DLC, feel free to start screaming over top of your cubical “Left 4 Dead DLC, hooray!”
Survival Mode will take place at the horde trigger points within each of the 4 campaign modes. You will jump between each of these points, (each more intense than the last) until you reach the finale titled “Last Stand.” Described as being the high point of the Survival Mode, “Last Stand” is a lot like horde – on steroids.
According to Valve “Survival Mode draws on the planning and communication aspects of a successful Finale or Crescendo event, while taking it to another level. It rapidly hits a fever pitch that only a well coordinated team will be able to successfully survive. Everything from simply covering a reviving teammate, to making every shot count to minimize reloads becomes crucial in Survival Mode.”
Infected Spawn Curve
Courtesy of Valve, above is a graph depicting the infected spawn time of Survival Mode. I would suggest tattooing it to your chest. Valve has also provided us with “lull” time. Which was described by Valve as “a short interval of time during which nothing new will spawn.” These are the moments that you can catch your breath, reload your ammo, and heal.
Source
