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Payday: The Heist - No Mercy revealed

Left 4 Dead crossover adds backstory to Valve's zombie shooter.

A trailer for Payday: The Heist - No Mercy has just gone live.

No official announcement has been made as to exactly what it is, but it looks like a DLC map set in Mercy Hospital from Valve's first Left 4 Dead.

Developer Overkill Software tagged the YouTube video with the following text:

"Have you ever wondered how the Left 4 Dead series began? It started with a heist!"

Overkill announced last month that a collaboration with Valve was underway concerning a Left 4 Dead/Payday crossover, though didn't provide details at the time.

We'll update when some more concrete information comes through.

Overkill's solid PC/PlayStation 3 cops and robbers shooter originally launched back in October 2011. See Eurogamer's Payday: The Heist review for more.