Resident Evil 6 ✕ Left 4 Dead 2

Resident Evil 6 ✕ Left 4 Dead 2 was an exclusive crossover on Steam for the PC versions of Resident Evil 6 and Left 4 Dead 2.

History
Sometime after Resident Evil 6 was officially announced for PC, Capcom had worked with Valve over the possibility of releasing a crossover between Resident Evil and the Left 4 Dead series. The crossover produced exclusive content for both games. The free content for each game was released to Steam on April 4th, 2013.

Resident Evil 6
The Resident Evil 6 expansion added content to The Mercenaries No Mercy gameplay mode. The main cast of Left 4 Dead 2—Coach, Nick, Ellis, and Rochelle—were added to the character roster, as well as two new enemies, the Witch and the Mini Tank. While playing as these characters, the music tracks used in the stages and results screen are changed to Left 4 Dead 2 tracks. These additions are compatible with the standard characters in the game, allowing the Resident Evil 6 cast to combat the same enemies and partner with the new characters.

Left 4 Dead 2
In Left 4 Dead 2, three enemy skins from Resident Evil 6 were added to the Steam Workshop as the "Official Resident Evil 6 Collection": Napad as the Tank, Ogroman as the Charger, and Lepotitsa as the Boomer. Featuring no official character skins in the collection, Valve encouraged users to add their own custom Resident Evil 6 characters to the Workshop.

Further notes

 * Each of the assets added to Resident Evil 6 were based upon preexisting characters and enemies: Coach is based on Chris Redfield, Nick is based on Leon Scott Kennedy, Ellis is based on Piers Nivans, and Rochelle is based on Sherry Birkin. Each of the Left 4 Dead 2 characters uses melee attacks, gestures, and some weapons shared by their Resident Evil 6 counterparts. Likewise, the Witch is based on the Bloodshot and the Mini Tank is based on the Napad.