The Aftermath Map Pack was a set of DLC for Resistance 2 released on March 26th 2009. The maps released were: Outpost in Red Woods, Orick, with Aftermath and Pit Stop based in Twin Falls, Idaho. The map pack cost $5.99 on the PSN at the time of release.
Also released at the same time as the Aftermath Map Pack were six character skins: a Black Ops variant, a Ranger variant, a female soldier, the Ravager, a Cloven soldier and Rachel Parker. These cost $0.99 each at release.
The DLC was available after download of the 1.4 update which introduced Meltdown to Competitive mode and Superhuman Co-op to the Cooperative mode. Although you need the map pack to play in these maps in the southern Orick map anyone can join even people without the map pack this could be due to a glitch it happens in unranked games and also ranked games.