Payday: No Mercy explains 'how the Left 4 Dead series began'

Update: Valve comments on the DLC.

Overkill Software promised it was working on a Left 4 Dead thing. Well, here it is.

The developer has released a new video, showcasing Payday: The Heist - No Mercy. There's no word on what it is exactly, but it appears to be a DLC expansion of SOE's online shooter.

"Have you ever wondered how the Left 4 Dead series began? It started with a heist," the video's description teases.

Mercy Hospital is a level from the first Left 4 Dead game, and is likely "one of the first places to have handled the Infected" according to Wikia. Could your actions in this DLC expansion accidentally cause the zombie outbreak in Valve's series?

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After the original video was pulled, Valve's Chet Faliszek offered a comment to Kotaku, saying the mission description is the result of "over-excited marketing guys."

"This is a 100% Overkill made mission for Payday: Heist simply set in no mercy hospital from the Left 4 Dead series. It is not telling the origins of the Infection and is not canon. There is a little cameo from one of the L4D characters (the comic should give you a hint) and a fun little easter egg with a payoff later in Left 4 Dead. Just us having some fun with a studio we are friends with and who have also made a great co-op game."