Rebellion Developments' Zombie Army Trilogy is about to get an infusion of experienced zombie killers: A free update today is adding the heroes of Valve's Left 4 Dead series into the shooter on PC, Rebellion announced today.

The Zombie Army Trilogy patch brings in all eight survivors from Valve's celebrated franchise: Francis, Bill, Zoey and Louis from 2009's Left 4 Dead, as well as Coach, Nick, Rochelle and Ellis from 2010's Left 4 Dead 2. That doubles the amount of playable characters in Zombie Army Trilogy, and allows players to pair a Left 4 Dead survivor with, say, Karl Fairburne of the Sniper Elite series.

"We're delighted to be able to bring such iconic characters to Zombie Army Trilogy," said Jason Kingsley, CEO of Rebellion Developments, in a press release today. "As thrilling zombie shooters designed to be enjoyed with friends, Zombie Army Trilogy and Left 4 Dead share a lot of common ground." You can see a trailer for the crossover above.

To mark the occasion, Rebellion is putting Zombie Army Trilogy on sale on Steam this weekend, dropping the price by 66 percent to $14.99 from $44.99. The deal will last from 1 p.m. ET today until 1 p.m. ET on Monday, Aug. 24. The company also released a free comic from British publisher 2000 AD to explain the Left 4 Dead crossover; you can check it out here.

Zombie Army Trilogy, which was released in March on PC, PlayStation 4 and Xbox One, packages together 2013's Sniper Elite: Nazi Zombie Army and Sniper Elite: Nazi Zombie Army 2 along with a third campaign.