Valve and Overkill have just revealed the collaboration they're working on: it's a heist level for Payday: The Heist called Payday: No Mercy, set in Mercy Hospital where, it appears, the bank robbers from Payday begin the infection that leads to the events of Left 4 Dead.
Brilliant. The trailer's below.
There's some interesting lines in there:
Valve previously denied this collaboration was a full prequel to us at E3.
See the PayDay 2 trailer here...