CW - Nightmare! Is a very difficult version of the base pack, it increases the difficulty by adding 20+ more zombie variants with progressive difficulty and a unnerving atmosphere. However the modpack changes so much that you probably won't even think its CW, hence why its referred to as "A Parallel Dimension".
Main Changed Things
An Ending
Point Blank Only
Completely Different Worldgen
Bleak & Unnerving Atmosphere
Many, Many more zombies
20 New Zombie Variants (As of V3)
Custom Cities
Many, Many, Custom items (Check the items tab for more info)
How does the game unfold?
Early Game, you spawn in a completely different bunker than in the base game, you escape it and the game starts, forget about making a base or anything, zombies evolve much faster and don't kill too many of em as they can alert hordes. The first days are nothing more but a race against the clock, as a lot of powerful zombie variants start appearing by day 20, and their power caps by day 50. Your main task is just to prepare, you can loot cities, structures and other things to give you loot which is sufficient for the overworld, but if you wanna go deeper you will need to make argent gunmetal which allows you to create much more powerful guns. Getting to the Depths different from the base game, but thankfully you have a quest book so that you can see everything you need to know about whats changed. You can consider yourself "Mid Game" after obtaining atleast diamond armor and some OK guns.
Mid Game, this is the part where you go to the Depths, the conditions for accessing the final boss are lenient, as it only requires you to kill about 12 mutants (obtain 3 nightmare stars), however if you want to have the slimest chance of actually winning, you will need to make hardened gunmetal, which is (Obviously) explained in your questbook. You obviously don't have to, but you really should, there are alternatives such as using the create mod to make some more basic Doom guns aswell as futuristic void armor, you should probably also make some baubles and reforge them. After obtaining the 3 nightmare stars you can craft a keycard, when you have made good guns and decent armor, you can go back to the starter bunker and open a specific door, this is where the end-game begins.
End Game, the endgame is rather short, you travel to an unknown place to defeat the boss which sits at the top of the structure that is the only point of interest in that hell hole of a dimension. The place is filled with top-tier enemies such as razorbacks aswell as futuristic shamblers, you don't have to kill them to progress, but you really should. After opening the final door, you will face the final boss, the fight is long and isn't considered easy. Defeating the boss drops a keycard, open the exit door and leave that dimension forever, for you have bested nightmare... or have you?
Hardcore Only Additional Tweaks
The hemorrhage effect reduces healing by 60% rather than by 40%
The cities now have much more powerful zombie variants
You lose your invincibility frames quicker
The Starting Bunker now has a couple of obstacles
The final boss gets new attacks and is generally more difficult
Custom Mutants have higher odds of spawning in the Depths