The DLC is called the Castithan Charge Pack, and adds the popular race from the SyFy tv show Defiance.
I haven't participated much in SyFy's experiment in having a TV show running alongside a shooter MMO, but at E3 I got to jump in to test some of the new content coming to the game. Fair warning: Defiance has middling shooter combat, and it's not exactly the prettiest game available on the Xbox 360. Still, the dedication to seeing the Castithan brought to life in the first of 5 planned DLCs for its first "season" demands some recognition and the additions add to the variety and fun of the Defiance game.
The Castithan - if you're not familiar with the TV show - are a race of pale aliens the other races view as arrogant or aloof, and they have a rigid caste system and social structure. The comparison to common depictions of Elves is apt - even the developer running me through the new content admitted space elves is what they refer to them as around the office. As a new playable race in Defiance, you don't gain any special abilities but you will have access to a specific questline that will give you more insight to the lore surrounding them.
Also added to the game from this pack are light sabers, er, charge blades. These melee weapons look pretty cool with a stylized blade of light, and can certainly be effective if an enemy gets close enough to you, while also providing special AOE jump attacks. Like other weapons in Defiance, you can modify and specialize charge blades to your exacting standards, including changing the color of the blade.
Vehicles are nothing new to Defiance, but this DLC pack adds a vehicle that can hold up to four players. There's positions for the driver, side gunners, and even a small seat in the back for the little brother tagging along on your Volge hunting trip.
Also on offer, is a new random open world event called a siege in which high level enemies assault an area on the map for a limited time. If you stumble across an area under siege, you can just hop in and start killing, there's no instancing or grouping required to participate. I got to try my hand at shooting some of the massive Volge attackers in a siege, and it was oddly compelling to just fight alongside my fellow defenders to hold the town.
Side note: The star of the Defiance TV show, Grant Bowler, booted me out of my demo of the game. He was very excited to try his hand at playing the game - for about five minutes - before ducking into the back room to meet with some executives.
Trion hasn't announced the pricing yet for the Castithan Charge Pack, but you could deduce a price point from the deal of the 5 DLC pack for $40. Expect the DLC to be coming soon.