Nightmare mode is only available to players who have achieved at least Rank 40. Nightmare mode is only available in Multiplayer and is used to get Elite XP, which is used for ranking up in the elite ranks (Ranks 41-50). To play Nightmare Mode, click the check box and the multiplayer/private match buttons would have "blood" on the the buttons. The zombies in Nightmare have 10 times more health, are faster, and deal more damage. However, there are slightly less zombies in nightmare than Singleplayer. Nightmare mode was also available in singleplayer until Februrary 27, 2012. Nightmare mode was also in SAS1.