Microsoft have announced that Halo 3: ODST will deliver a completely new campaign, a new cooperative multiplayer experience called Firefight, the complete Halo 3 multiplayer experience on a standalone disc, and new competitive multiplayer maps for Halo 3.

Microsoft and Bungie today also revealed the remaining three multiplayer maps for ODST; Heretic, Citadel and Longshore:

• Heretic returns players to familiar ground as the “Halo 2” arena Midship makes its long-rumoured début in Halo 3. The stark interior of the Pious Inquisitor might appear austere on first glance, but Heretic’s symmetrical spaces require players to execute complex strategies in order to outplay their opponents.
• Citadel may be a long abandoned Forerunner stronghold, but its small, symmetrical confines are far from secure. Two opposing bases offer little safety for players looking for solace. Instead, Citadel forces small squads to stay in constant communication, keep their heads on a swivel, and be prepared to adjust combat tactics in close quarters.
• Longshore lacks the commercial ships that once made it a lucrative port for Old Mombasa, but it still bustles with activity. Designed for large scale combat and well suited for smaller affairs, Longshore’s multitude of industrial buildings, elevated walks, and open inlets conspire to create a complex battleground tailored for multiple combat scenarios.
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In addition to a new campaign, Halo 3: ODST will introduce a new cooperative multiplayer mode called Firefight, which allows Halo fans to fight as members of an ODST squad against waves of increasingly challenging enemies across a variety of battlefields. Halo 3’s “skulls” will progressively activate as time goes on to alter the gameplay and up the challenge.

Halo 3: ODST is due this Sept. 22 exclusively for Xbox 360.