Obviously this DLC will focus on vampires. Hunting Vampires, or even being a Vampire. The plot, judging by the trailer, is that a vampire lord is seeking the Elder Scrolls to black out the sun. Meanwhile Dawnguard, an elite force of vampire hunter, is attempting to stop them from achieving this goal.
Your choices are to become a vampire and work towards eternal darkness, or just the Dawnguard and work towards killing the vampires and preventing eternal darkness across the land.
There will be new skill trees for werewolves and vampires. If you choose to side with the vampires you will have increased health, magic and stamina by 50% and the ability to transform into a vampire lord anywhere in skyrim.
Usually in Skyrim if you're a werewolf you can not become a vampire and visa versa but it has been confirmed that werewolves can now become vampires. New powers while being a vampire lord include twirling enemies into the air and summoning a gargoyle. Other powers include paralyzing enemies and adding poison damage to attacks. Becoming a vampire seems to make you incredibly powerful. We are still not sure if you will be shunned by townspeople if you're a vampire or not but the second we know we will let you know!
Vampires constantly need to feed to survive so you need to do this quite often to survive, and this will be for the rest of your time playing Skyrim if you choose this form. However, the trailer suggests you can also feed on the dead so perhaps you won't need to kill any innocent civilians if that isn't your style. Vampires also have a heightened sensitivity to the presence of nearby creatures, thus making it easier to hunt from now on. You can also turn in to a cloud of bats to escape dangerous situations and to warp yourself around the Skyrim area.
Should you choose to join the Dawnguard you will get new weapons to help you hunt and kill vampires, plus some pretty strong armour to boot. By siding with them, you can tap into their blacksmithing knowledge and make some of these items yourself. You can also call upon an armored troll to fight by your side, which sounds pretty awesome! The vampires may be strong but they do offer a good balance for you to decide which to be, althought I can only assume anyone who is getting this (and willing to cough up those points to do so) is purely because you can turn into a cloud of bats and be an insanely overpowered vampire!
The expansion introduces a few new features previously unseen original content, including new enemies like vampiric giants and those that turn into bats (a skill you will also have), new weapons like crossbows, a new flaming mount, and many more. Archers would be happy to know that crossbows hit harder and potential knock-back effects, but reload more slowly. A new dragon shout is also available which allows you to steal someones soul and turn them into your minion.
The DLC will provide roughly an additional 10-20 hours of content and is integrated into Skyrim's existing world and it will be available on the Xbox 360 on June 26th for 1600 Microsoft Points. Its release on the PC and PlayStation 3 will be delayed by at least a month.