The fourth multiplayer DLC for Mass Effect 3 codename Retaliation adds a new enemy faction to the game: the Collectors. You will be able to play with 2 new classes: Turian Havoc Soldier, Turian Ghost Infiltrator, with many more being added on a regular basis, each with new powerful abilities and melee attacks.
Checkout the new "hazard" versions of the existing maps, adding Acid, Lightning, Meltdowns, Sandstorms, Swarms and Whiteouts! Besides that there are new unlockable weapons, powerful upgrades and a whole new Multiplayer Challenge Feature.