Update 2:

Valve confirmed to IGN that the work being done on this Payday campaign will not tell the story of the Left 4 Dead outbreak. A Left 4 Dead character will make a cameo, but it's not meant to be a substantial extension of the Left 4 Dead fiction. Overkill is handling all development on the project.

After posting this story, it appears the original YouTube video was moved to 'private' status. IGN has reached out the Valve and Overkill for additional details.A teaser trailer popped up on the official Payday: The Heist YouTube channel showing off a collaboration between Valve and developer Overkill. Titled No Mercy, the in-development content will allegedly tell the story of how Left 4 Dead's zombie outbreak began. "Have you ever wondered how the Left 4 Dead series began? It started with a heist!" says the YouTube video description.From the looks of things, it involves a group of armed thieves who assault Mercy Hospital in search of a particular patient's blood. Naturally, the search is not a peaceful one.Payday: The Heist launched in 2011 for PlayStation 3 and PC, and featured a style of gameplay similar to Valve's Left 4 Dead.