Maybe an item that attracts nearby mobs upon rightclick (maybe skeletal bane? since you can easily pwn skeletons with it) and also attracts items while in the hotbar. and also a new armor piece called glasses of revealing that gives you the ability to see nearby mobs through walls. Also a ton more ideas below.
Dark Sword- sword that drains your hunger faster, requires the core and maybe a new item called Nightmare Fuel. Has very high damage, insane durability, and has a 10% chance of instant damage inflicted on user each hit.
Nightmare Fuel- Created by a new block with possibly a 3D model if not then it's an item, the shadow manipulator. And upon right-clicking drains all your hunger, and gives you poison II. Why go through such a dangerous process? cause of Nightmare Fuel. Used as a crafting ingredient, and a power source for any other endgame stuff you or I may think of.
Nightmare Armor-Armor made using Nightmare Fuel and a core, this armor absorbs all incoming damage, has incredible durability, but you have slowness 1 and your hunger drains faster while wearing it. Armor buffs only applied if wearer has full set on. May also have the power of flight while equipped.
Oh and the instant damage debuff of the dark sword is amplified. An intense nightmare is fatal, right?
Heat Stone- A magical flame, encased in a impenetrable container. Upon right-click it has the GUI of a furnace, but smelts at almost-instant speeds. Still the normal fuel, but has an extra slot for nightmare fuel. Smelts items instantly using the same amount of fuel, but consumes 1 nightmare fuel per half stack.
Wait, there's more!
Fan of the Wind-It's a terrible name but it's basically a fan, which repels mobs and items. Simple as that.
The Hilt of the Blacksmith-An extraordinary item which compels the wielder's darkest thoughts in order to slay enemies. Hitting a target causes 3 ghost pigs ( better than lame killer steves, right?) to start destroying the target. Pigs disappear after target is dead, or they are dead. Pigs have VERY low life. about 5. Pigs has no knockback unlike the attack pigs.
The Door of the Cartographer: Another sadly terrible name, this door acts as a portal to possibly-unexplored areas, used in searching mod ores or important structures that wasn't generated in an existing world (A problem I have had for ages ) Or possibly just for looking for new fascinating areas. Acts as a normal door at first, but on contact transports you 100 chunks (1600 blocks, am I going insane?) in that direction, and placing a return door at that location. Door exactly teleports you 1600 blocks in the direction the door is facing, not caring about whether the exit door is floating or underground, in which it needs to dig a spot for the door. maybe door acts as an anvil and has a chance of being degraded/destroyed each time you use it.
To amp up the endgame-ness of the Nightmare set, you need to craft 9 nightmare fuel into a block of horrible dreams, and smelt that for an ingot. The ingots can be then crafted into a block of pure terror, then those should be used for the recipes.
Oh, and a harder way to get nightmare fuel to prevent just spamming killing yourself:
You still lose all hunger and have poison II, but this time you need to face 3 hallucinations. Hostile pigmen conjured by your insanity, these mobs are considered passive, to prevent switching to peaceful to safely get nightmare fuel. These terrible convoluted abominations of pure terror are a team composed of the following:
2 melee pigmen, which wield dark swords and have smoke particle effects, and an archer pigman with the same particle effects. all of them are hostile to you at first, but they are extremely slow, so you can kite them towards the archer so they'll turn on each other. Each pigman drops 2-3 nightmare fuel, and a VERY small chance of dropping a low-durability dark sword.
Oh, and screw the poison II, and replace that with a blindness effect for a slender-like feel while fighting.
Perhaps the life extractor should be upgradable with diamond blocks, nether stars, or other things?
Edit: Just thought of something. Maybe you could add a dandelion of doom, that rarely spawns on tall mountains, or one of those floating glitch thingies in extreme hill biomes. It would suck the life out of everything around you, including plants, players and mobs. If a player is wearing a Nightmare set previously mentioned by yarethegoodnamestaken, they would be exempt from these debilitating abilities.
AnotherEdit: Perhaps you could add some AntiGravity Feather? It would levitate you and the blocks around you. Floating house for the win!
P.S. I really like mods that add random magical utilities.
Hey, just wanted to let you know that I made a mod showcase of this mod.
I've got 6000+ subscribers, so hopefully this will also bring a bit of attention to your mod.
I love how well balanced this mod is, with the crafting recipes being expensive as they are.
Anyway, here's the video. Feel free to use it for anything you want .