"Halo 3"s story or campaign mode can be played through alone or cooperatively with up to three other players via Xbox Live or System Link. Instead of having each player be an identical character as in previous "Halo" games, the first player plays as Master Chief, and the second player plays as the Arbiter. The other two players control two Covenant Elites, N'tho 'Sraom and Usze 'Taham, each with their own backstories. No matter which character is played, each player has identical abilities, though their starting weapons vary. Hidden skulls found on each level cause changes to the gameplay when enabled, such as giving the enemies extra health, changing in-game dialogue, or modifying AI behavior. These skulls, as well as the difficulty level and the speed at which the level is completed, provide multipliers to the total score. Players are awarded gamerscore points for unlocking Achievements by reaching a certain score in each level.