Valve and Capcom are forming an undead alliance as they plot to merge the worlds of Resident Evil and Left 4 Dead.The crossover is specific to PC, but will see Left 4 Dead 2′s four protagonists invade Resident Evil 6′s Mercenaries No Mercy mode through a free download on April 5 Ė the PC version of Resi 6 is due for release March 22.The gameís enemy roster will also be bolstered with L4Dís Witch and Tank infected variants. Left 4 Dead 2, meanwhile, will add three as-yet unspecified Resi enemies through the Steam Workshop sometime later this spring.