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For Halo 3 ODST there are Achievements For halo 3 ODST itself (click this link to see them: /wiki/Achievements: scroll down until you see the ODST Achievement list,) but there is also a Halo 3 multilayer disk (which is it's own disk with all the new maps plus 3 new ones.) The multilayer disk will let yo unlock achievements due to it having matchmaking with it.

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