Onslaught adds objective-based gameplay to four maps: Valparaiso, Atacama Desert, Isla Inocentes, and Nelson Bay. Here, up to a squad of four players must secure objective points and battle waves of CPU-controlled enemies. Onslaught mode is currently available on the Xbox 360 and PS3 for 800 Microsoft Points and $9.99, respectively.
Update: Looks like PC Gamer slipped up, as DICE's community manager Daniel recently said on Twitter the article "is not accurate" and that DICE is "currently researching if Onslaught will be on PC. No release date." We'll keep you posted.