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Halo 3:ODST is a first person shooter and was released on September 22nd 2009. It takes place during the events of Halo 2, more specifically right after the Halo 2 level, Metropolis. You play as an ODST nicknamed the Rookie, in place of the Master Chief. Halo 3: ODST is a stand alone expansion to Halo 3.
Since you do not play as the Master Chief the gameplay mechanics have been tweaked from Halo 3. You lose the benefits of the Mjolnir Mk. VI Powered Assault Armour.Shielding - you do not have a regenerating health bar but a stamina bar that reacharges and a health bar that doesn't recharge. To regain health you need to pick up one of the health packs distributed around the city (it is similar to the health system in Halo CE). A couple of other minor gameplay changes include the inability to dual wield weapons and restricted jump height and movement speed. You have access to an in-game PDA and a night vision function, which is toggled by the X-button.
Firefight: A new way for the Halo multiplayer to be played. Fire-fight is a game mode where you can play with up to two players on the same console or up to four players over Xbox Live or system link. After gathering a few of your friends or getting some pizza and a drink and your ready to battle wave after wave of Covenant forces with skulls activating through out the game to make things more... interesting. thumb|300px|right|video intro to ODST
There is no Forge mode featured in the game but Theater mode makes a return.
Halo 3: ODST comes with two disks, a copy of the Halo 3: ODST disc and a copy of the Halo 3 Mythic Multiplayer disc which contains all the map packs currently on Xbox Live Marketplace. Halo 3: ODST also acts as the exclusive gateway into the Halo: Reach Xbox Live Multiplayer Beta. If you had pre-ordered the game at selected retail outlets, you received an exclusive redeem code to unlock Sgt. Johnson in the Firefight mode. (For more information visit Bungie.net)