The Battlefield: Bad Company 2 Onslaught Trailer was a teaser for the Battlefield: Bad Company 2 cooperative DLC, Onslaught.
"Battlefield: Bad Company 2 Onslaught mode can be played with up to four players across four multiplayer maps Valparaiso, Atacama Desert, Isla Inocentes and Nelson Bay redesigned with new lighting, time of day, added vehicles and other effects. Each map has a dedicated gameplay focus, requiring different levels of teamwork in order to complete the objectives against an onslaught of enemy AI. Vehicle warfare comes to the forefront in Atacama Desert while Nelson Bay focuses squarely on infantry assault. Players can compete in squads and check individual progress via the dedicated Onslaught leaderboards."