New Skyrim Dawnguard Details

As said before the new DLC allows you to choose between a Vampire legion or the Dawnguard who are set to destroy the vampires.

All this content is integrated into Skyrim itself so you can use crossbows and your new Conjured creatures all across Skyrim as well as become a vampire lord all across Skyrim as well. It is said that one of the new castles is located north east of solitude which you can learn from a Guard's rumours.

If you choose the side of the Dawnguard you don't get to have a new form like the Vampire lord but they do give you the ability to cunjur an armoured troll (Which i hope is a thrall) as well as access new blueprints to build new armour and Crossbows.
The Dawnguard specialise in Armour and weaponry so expect some good pieces of armour by the end of their quest.
Crossbows do more damage than normal bows but take longer to load to fire.

If you choose the side of the vampires they will allow you to become a Vampire overlord at will. With this new power you can turn into a winged snarly type creature that can turn into a flock of bats and hover over environments at will. If you are a werewolf you will still have the chance to become this vampire but you will have to give up your wolf ways. But if you dont like the Vampire Lord you can always change back to a warewolf afterwards.
This Vampire mode does give you a whole new perk tree though and it has also said that a new perk tree will be given to the werewolves as well.
As for benefits to being a vampire, You are able bite and kill enemies which gives you new set of skills specific to the vampire lord. These skills include the ability to Cunjur a gargoyle to fight for you, or you can also Choke, Paralyse and Pull enemies towards you from a distance with new Vampire lord specific spells which you get from perks and killing other people.
You can also Detect all creatures around you and add poison damage to all your attacks.

As for new lands you get a whole new neither realm which is known as Soul Cairn which is filled with all sorts of new enemies and areas as well as lots of purple fire. It also features all sorts of Fiery horses that you can ride across the land. It is also said that these new areas have hidden elder scrolls which will probably affect the quest line so look forward to a in depth story of at least 15 hours of play.

So far the DLC looks great and it is said depending on what side you choose the content will be altered so make sure to make a second play through if you want to get the best out of the DLC.