In the game's campaign mode, players explore the ruined city of New Mombasa to discover what happened to their missing teammates in the midst of an alien invasion. Bungie initially conceived ODST as a small side project to produce in the lull between Halo 3's completion and Halo: Reach. The game was the top-selling title in the United States in September 2009, and sold more than 3 million copies worldwide. The game features an open world environment in the updated Kenyan city of Mombasa, referred to as New Mombasa. Although the gameplay of ODST bears a strong resemblance to that of previous Halo titles, the player does not assume the role of the enhanced human supersoldier Master Chief, protagonist of Halo 3. As a lone human soldier known as "the Rookie," the player's goal is to discover what happened to his missing teammates. Adding to the difficulty are modifiers called "skulls", which give enemies new abilities or a handicap to the player – the "Catch" skull, for example, causes enemies to throw greater numbers of grenades.
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