The Story Line

The story of The Elder Scrolls V: Dawnguard involves the return of a Volkihar vampire lord named Harkon, who seeks to end the tyranny of the sun. Harkon intends to use the power in the ancient Elder Scrolls to accomplish his goal, while an order of vampire hunters known as the Dawnguard seek to stop him .
The Dragonborn is given the option either join the Dawnguard and help defeat Harkon or join forces with the Volkihar vampires.

Dawnguard can be started by listening for rumors or asking Hold Guards about a group of vampire hunters who are recruiting. This will steer the Dragonborn in the direction of Fort Dawnguard and the add-on will begin.

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Alignment

Vampires
Joining forces with Harkon allows the Dragonborn to become a Vampire Lord - the paragon of the species. If the Dragonborn is already a Werewolf, accepting to become a Vampire Lord will remove the Lycanthropy, but it can be regained later. The transformation process works similar to Lycanthropy and grants access to unique powers. One ability is confirmed as Vampiric Grip which suspends an enemy in the air with telekinesis as their health is drained. Other powers and abilities include summoning a gargoyle, hovering above the ground and transforming into a cloud of bats.
As with Lycanthropy, townspeople will attack a Vampire Lord on sight, however this will no longer happens if the Dragonborn is a regular vampire. Unlike werewolf transformations, the vampire form can be exited at will.

In addition to new powers, joining the vampires grants access to Castle Volkihar - an island stronghold that not only serves as a base, but grants bonuses to vampiric powers and gives blood potions, which heal and count towards vampiric feeding.

DawnGuard

Joining Dawnguard grants the Dragonborn access to Fort Dawnguard, a large keep said to be hidden somewhere in the southern region of Skyrim [4]. The unique benefits of joining the Dawnguard include various new weapons and armor to fight vampires, including crossbows. Furthermore, Dawnguard members will also be able to "hire" Armored Trolls, which serve as temporary followers.

Perks

The Elder Scrolls V: Dawnguard introduces two new perk trees: Vampire Lord and Werewolf. Rather than spending points to add perks as in the base game, new perks are gained by feeding on enemies with the new powers. For Vampires it's consuming life blood through a bite power attack or a Drain Life spell. For Werewolfs it's consuming the heart when feeding on a dead corpse. This approach allows high or maximum level characters to obtain new perks that would otherwise be difficult or impossible.

Vampire Lord
The Vampire Lord skill tree includes eleven perks
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Perk Description Requirements
Power of the Grave 50 point bonus to health, magicka and stamina as a Vampire Lord.

Detect All Creatures Power of the Grave

Mist Form Turn yourself into an invulnerable cloud of mist while regenerating your health, magicka, and stamina. Detect All Creatures

Supernatural Reflexes You move quicker, while enemies move slower. Mist Form

Blood Healing Killing a person with a power attack bite restores all your health.
Power of the Grave
Unearthly Will Night powers and Blood Magic cost 33% less. Power of the Grave
Poison Talons Melee attacks deal 20 points of poison damage. Unearthly Will/Blood Healing

Night Cloak A shroud of bats feed on enemies within melee range. Poison Talons

Vampiric Grip Blood Magic: Can pull a creature to you from a distance, and do choking damage once it's close. Power of the Grave

Summon Gargoyle Summons a Gargoyle. Vampiric Grip

Corpse Curse Paralyze your foes with this magical blast. Summon Gargoyle

Werewolf
The Werewolf skill tree includes eight perks

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Combat

Dawnguard introduces several new additions to both ranged and melee combat. The first new addition is the inclusion of Dragonbone weapons that are reported to be the stronger than Daedric Weapons[8]. Mounted combat is also included, which allows the Dragonborn to fight on horseback with a variety of weapons, but not spells.
Crossbows are added to the arsenal of weapons at the Dragonborn's disposal. Crossbows will remain loaded while running through the world and provide a quicker shot. They are, however, slower to reload than bows. Another benefit is that each shot has a chance to stagger an opponent. All of the perks spent in Archery skill tree will apply to crossbows . Crossbow bolts can also be crafted with various enchantments, such as fire and frost damage.
New shouts are also included in the add-on. One shout called Soul Tear can be learned from Durnehviir, an undead Dragon found in the Soul Cairn. This shout allows the Dragonborn to steal the soul of a defeated enemy and raise their soul as an undead minion.

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