Only attempted by the foolish and the stupid. Nitemare difficulty increases the health of all zombies by 300%. Damage is doubled, and enemies bypass player armor. Bosses gain the ability to cast Shadow Step, making them incredibly fast. Additional Weapons will not spawn, forcing players to make due with only the Gauss Rifle, Flamethrower, Sniper Rifle, Pistol, and Laser Rifle.
This gamemode can only be triggered in the first 666 seconds of a campaign, and is not compatible with Survival Mode. The team must have a collective Squad Rating of at least 1950. Any less and typing -ns will cause all players to explode into a bloody pulp. In addition, players can only use 1 of each class, forcing them to attempt the Expeditionary Medal. Automatic reload length is three times normal length, and manual reloads are 50% longer as well. Every hit also gives an Open Wound, and the chances of getting Cripple and Short Circuit is greatly increased.
Good luck soldier.