With the release of the 7th Edition Army Book, the Dark Elves are no longer just an evil counter to the High Elf army.  Where the High Elves received the benefit of extra Special and Rare unit Choices, and the obvious 'Always Strikes First' rule, the Dark Elves received little to keep them competitive to their goody-two-shoes cousins.  The Dark Elves benefited from the upgrade of the Hydra and Black Guard (making them much tougher while somehow making them cheaper points-wise) and the ability to deploy assassins above and beyond the Character quota in the army.  Lastly, their magic was stimulated by allowing Sorceress' to use more dice than other armies (and gave them a Dark magic spell to generate more dice).

 Click on the Dark Elf icon for the Army List

Knowing where Games Workshop's book designers wanted us to go, I added the hydra, Black Guard and a pair of assassins to the army.  However, unlike the High Elves, the deadly Reaper Bolt Throwers share a Rare slot with the uber-powerful Hydra.  Thus, you must choose between them.  If the army is not successful in the tournament, my hunch is to add more magic and convert the hard fighting units over to cheaper spearmen (using as many models as possible to take up space while you pummel the enemy with overpowering magic).

Never the less, by putting so much emphasis on the elite units that make up the Special and Rare choices for the army, this list only comes in scoring 13 out of 20 for Army Composition (the lowest mark so far) and 8 points for price (also the lowest mark so far) since it is estimated to cost $550 for the complete force.  Thus, there is plenty of room for improvement should the army not fare well in the tournament.

On an aesthetic level, the Dark Elf army is a nice looking army if you go with the traditional purple and blue paint scheme.

As your best Core unit choices, the lead off is to purchase 2 units of 5 Dark Riders and equip them with repeater crossbows.  These are very effective in the role of Fast Cavalry and can also be used as War Machine hunters since their opening move of 18" will put them within charge/crossbow range of any war machine.  Without the expense of the repeater crossbows, they are also the throw-away unit for your army.  So, if you choose to shift around your configuration and need the Special Unit slot occupied by the harpies, simply use your Dark Riders in this role (and don't be afraid to dump the unit of crossbowmen for a third unit of Dark Riders).

The Reaper Bolt Throwers are the must have unit in the army.  You can get two for a single rare slot, which means a pair of these can dominate a battlefield in battles where the two armies are under 1,000 points strong.  I am still thinking that this army could be even more effective without the Hydra and fielding 2 more Bolt Throwers, but for keeping your army collection balanced, it is best to limit yourself to a pair and use the Hydra until you have completed this collection.

Since magic is an expected and potent weapon in the Dark Elf army, you should field your two Sorceress' as soon as possible.  Dark Elves have a base leadership of 8, which is respectable, and the elite units being used have a 9.  Therefore, I wouldn't worry too much about having a Leadership 9 General early on in the process.  Those that need it most (the harpies and Dark Riders) will be out of your 12" command radius anyway.

The Black Guard is the unit that Games Workshop wants you to play with this Edition.  They used to be a 0-1 limited Rare unit, now they are a Special Unit with no limitation on how many can be fielded.  They made them 3 points cheaper, all the while giving them 2 attacks each, making them Immune to Psychology and re-rolling rolls 'to hit.'  Although it may be desirable to field two units of Black Guard in this army, I leaned toward a unit of Executioners so that you won't have too much money invested in Black Guard if they decide to go back to limiting them to 0-1 in the next edition.

The Dark Elf crossbowmen are an under-estimated unit in the Dark Elf army.  Yes, the repeater crossbow doubles the cost of a regular Dark Elf warrior, which is why we try to shoot with them as much as possible.  However, by spending the extra point to give them a shield, they are capable warriors (WS 4) in close combat with a hand weapon, shield and light armor combination that produces a 4+ armor save.  Thus, for 100 points they can be a valuable unit to protect the flanks of your more expensive elite units.

Like the Black Guard, Games Workshop wants you to field assassins.  You are entitled to field one for every foot unit in the army (that would be a pair at this stage).  The Rhune of Khaine is a mandatory upgrade as it gives the assassin a forbidding +d3 attacks to his profile.  I also like giving one the Cloak of Twilight, which enables him to fly around the battlefield and essentially choose his targets.  This essentially makes this assassin an effective war machine hunter.

Harpies are a cheap, fast unit and have been chosen as a throw-away unit that can also fulfill the role of War Machine hunter.  Never the less, with the capabilities of other units in the army, these will be the first to get removed if points are needed elsewhere or the army configuration changes.

Early on you can spring for a Dreadlord on a Cold One mount to be the army's top warrior.  He could also function as your General, either keeping him with your battle line and using him to charge in and swing a combat one way or the other, or by sending him around the flanks with the Dark Riders and using his leadership to assist them.  You will be using the rider to convert for the Dreadlord on the Dark Pegasus, which happens at the 2,000 point threshold.

If you choose to field a unit of Executioners, you must spring for a Death Hag, make her the Army Battle Standard Bearer and give her the Banner of Hag Graef.  This allows the unit she is with to Always Strike First.  Executioners have great weapons and a paper-thin Toughness of 3.  With only an armor save of 5+, they seldom get to strike back against an enemy who they go up against.  Thus, the Banner of Hag Graef is a requirement with the unit and only a Death Hag can carry it (since unit standards cannot exceed 25 points and the Executioners are Khainite).  To make the most out of what you have, I would use the torso and spear arm from the Morathi model (which you will be purchasing for the Dark Pegasus) and assemble it on a pair of extra legs from the plastic Dark Elf warrior unit box.  Another alternative would be to seek out a model that could be used specifically in the role, like the standard bearer from the Witch Elf unit (if it can be bought individually).

The debate about whether or not to use the Executioners falls on what else the army has to deal with heavily armored opponents, like units of knights.  The standard Dark Elf warrior has a Strength of 3 and the Black Guard have a Strength of 4.  This means that most knights will still have a 3+ armor save or better against them.  Therefore, I believe the Executioners are necessary to fulfill the role of "can openers" in the army. 

By now you probably have all the components you need to convert the Dreadlord on the Dark Pegasus.  Essentially it is the Character on the Cold One (I used Malus Darkblade) and Morathi's Dark Pegasus mount.

The War Hydra is the linebreaker in your army.  Due to its high toughness and ability to regenerate, it can absorb a lot of damage as it pushes forward with (or ahead of) your battle lines.  In addition, it has 7 attacks (along with 3 each from the Beast Masters) which makes it a formidable offensive presence in the army as well.  If it weren't for the sharing of a rare Slot with a pair of Bolt Throwers, I would recommend two of them.  But, the Reaper Bolt Throwers are more important to the balance of the army and give you much needed firepower against large targets (and knights) on the other side of the battlefield.