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On Halo 3, the skulls may only be activated whilst playing Campaign. The menu is opened when you press the "X" button whilst in the campaign lobby. There are 13 skulls in Halo 3, of which 9 are Gold that unlock achievements, and also 4 Silver skulls which do not. The skulls help in achieving the Campaign Meta Score, by increasing your score multiplier.

Hope this has helped answer your question

Cameron (:

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