 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 <unk> ' <unk> and <unk> ' <unk> , 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 <unk> points for unlocking Achievements by reaching a certain score in each level . 
