Here is a list of items that are exclusive to nightmare mode and can't really be found in other versions
Axe Of Quake
Defiblirator Cables
Futuristic Void Armor*
Horde Caller
Heart Shard
Hardened Gunmetal Ingot
Nightmare Star
Nightmare Spawnpoint
Ranger's Armor
Razorback Sword
Thunderbolt
[ Axe Of Quake ]
Is the best melee weapon, doing up to 20 dmg in one swing, however it's also very difficult to craft, requiring 4 Hardened Gunmetal, 1 Netherite Ingot and 2 Nightmare stars. Additionaly when hitting a mob you'll gain an Absorption Shield, it gets overriden when you land a new attack. The weapon also has a lot of enchantment slots so you can customize it a lot.
[ Defiblirator Cables ]
The cables are an item that drops from mutant zombies only in the void dimension, having a 100% drop chance, if you obtain 4 of them, you can craft a defiblirator (totem).
[ Futuristic Void Armor ]*
The futuristic void armor is a semi-nightmare mode item, since it can also be obtained through commands in regular CW, under ./give (player) the_deeper_void:futuristic_void_armor_(piece), it was originaly created as an additional armor set for the official CW server, to be more powerful than juggernaut since protection caps at level V. Each piece of the armor grants bonuses
Helmet - Low-Gravity Switch (Nightmare Only)
Chestplate - Immunity to Blast Damage
Leggings - Immunity to Fire Damage
Boots - Immunity to Fall Damage
Full Set Bonus : Immunity To Non-Penetrating [TACZ] Shots
Since tacz isn't in nightmare, the full set bonus is quite useless, however it can make you an unstoppable force on the official server.
The armor has a crafting recipe in nightmare mode, utilizing Xan Chips, which require you to pass the majority of the create mod to obtain, they can later be combined with netherite armor, netherite sheets and steel to craft pieces or armor. Also small trivia : the armor has the same model as the armor of the final boss.
The armor can't be repaired, and is reforgeable with steel.
[ Horde Caller ]
The horde caller is a special block, made out of 72 Rotten Flesh and a bone block, upon being placed, it will summon a horde in about 8 ~ 11 minutes, after that it will get consumed, it's a great way of spending all of your redundant rotten flesh.
[ Heart Shard ]
Drops from mutants with a 25% drop rate, an alpha mutant however has a 100% drop rate of it. Upon collecting 5 of these shards you can create a heart container, which will give you extra health (up to +3), this item was mainly added because Solonion (Spice of Life Onion) had bugs related to HP scalling.
[ Hardened Gunmetal Ingot ]
The Hardened Gunmetal Ingot is an item thats used in a lot of end-game item recipes, it is crafted by fusing steel, rhenium-tungsten alloy, rhenium-titanium alloy, and argent gunmetal.
[ Nightmare Star ]
The most important item in nightmare mode, only drops from mutants in the void, with a 25% drop rate, has a 100% odds of dropping from an alpha mutant. The Nightmare Star is used in countless nightmare mode recipes, mainly for the keycard that allows you to finish the game. It can also be turned into a regular nether star if you add 4 diamond blocks and 4 iron blocks to it in a crafting table.
[ Nightmare Spawnpoint ]
The nightmare spawnpoint item is a replacement to the bed, since you can't sleep in nightmare mode. You can craft it with a redstone blocks, 5 steel ingots and 2 redstone torches, multiple players can bind their spawnpoint to one nightmare spawnpoint block.
[ Rangers Armor ]
Rangers armor is a nightmare mode exclusive armor set, it's better than juggernaut, and unlike futuristic void, you don't have to bear through the entirety of create to obtain it. Also its based on quake (1996).
The armor can be repaired with hardened gunmetal, and reforged with steel ingots.
[ Razorback Sword ]
A very secret and rare item in nightmare mode, having a slim 7.5% odds of being dropped by a razorback. It deals 12 base damage, can't be enchanted and is unbreakable, when hitting something with this item, it will generate an explosion, doing splash damage and thrusting the receiver atleast 10 blocks up, its a great way of dealing with very fast zombies.
[ Thunderbolt ]
The Thunderbolt is a special weapon, it is the true end-game weapon. It has an rpm of 1200 and a damage per shot of 15, making it melt anything it touches, however its very expensive to craft and to fire, as 200 rounds cost 100 redstone, 10 gold and 20 brass. The thunderbolt can also be obtained by defeating a secret boss. (also yes its from quake)