Why? I'm glad they included this item, simply 'because', but don't expect to see it used. Especially not by Dark Elves.
Fozzrik's Folding Fortress
Not really sure what to think of this. I'm not sure there's going to be many good uses for this, although for some crazy shooting army this might actually be a pretty good item choice. 100 points is hard to swallow though. You could use it to cut off some movement path, I suppose.
For the same cost, you can take a Dark Pegasus, which has its own advantages, and lets you take other magic items. Don't bother.
Sadly, just not so good anymore. Terror doesn't work as well these days, and since BSBs can let you re-roll the test, it may never work. Don't spend your points here.
Too many points to be useful. You'll need to spend those points on making your character more useful in combat if you're already in range to use this.
Crystal of Midnight
You're very likely to see a wizard of some sort out there, and since most wizards aren't Ld 10, this may actually be worth its points. It's a bit random though, as it removes a random spell if it even goes off. I'm going to say you're probably not going to use this, but might be a nice trick to throw in from time to time.
Crown of Command
I love this. Ld10 and Stubborn for your Dreadlord means he can go out on his own and cause some damage without fear of being run down by combat resolution. There are a lot of builds that this comes in handy for. It even makes running a Dreadlord on a Dragon potentially useful!
Black Dragon Egg
Given the new rules for breath weapons, this seems to be everyone's new favorite item. I'd recommend throwing it on a character on foot if you can fit it in. It's a decent amount of free hits at a high initiative. Combined with a combat with another monster in it, or the Armour of Blood, it's going to be very valuable.
Potion of Strength
This is the one from the Dark Elf book, which is 10 points more than the one in the BRB. Good bet, you'll choose the other one. However, this item gives you the benefit of choosing later on in the turn, AFTER you know that you're in combat already. Is that worth 10 points? Probably not.
Gem of Nightmares
With the changes to fear, I just don't see a point anymore. Don't waste your points.
A handy item, but too costly to take in most armies. Your Dark Elves are not likely to lose wounds in small enough number to get a chance to use this.
The Guiding Eye
It's not an exciting ability, but if you really want to run a huge shooting block, it's certainly worth its points. There are some army builds I could see finding a place for it, but not many.
A pretty costly item for a benefit that isn't likely to come up much. Will be worthless in most games.
Ruby Ring of Ruin
I actually quite like this item. It's a good points value, and even though the rules for bound items have changed, it's a 2/3 chance to get a free spell off that can chip away at your opponent. If I have room for this, I will probably take it from time to time.
Terrifying Mask of Eee!
Like the Deathmask (but so much cheaper), it's just not worthwhile in this edition.
Potion of Strength
Cheaper version of the formerly Dark Elf-only item. There are a lot of times where the extra strength comes in handy, so I would suggest throwing it on a Dreadlord (with an extra attack item) if you think he's likely to get in combat. 5-6 Strength 7 attacks can put a hurting on big things.
Potion of Toughness
Although it makes your character harder to kill for a turn, you have better ways to accomplish that task. Don't take it.
The Other Trickster's Shard
Pretty nifty item, and worth taking on a hero character from time to time. Against Daemons it's going to be VERY frustrating.
Very inexpensive, but almost worthless. If you really have nothing better to spend the points on, you may as well take it.
Potion of Foolhardiness
Not much use- you have good ways to make yourself resistant to psychology for the game. Not a big concern for a Dark Elf player.
Potion of Speed
Dark Elf initiative is generally high enough that this item won't get the jump on any opponents. Not useful.