The story focuses on a vampire lord called Harkon who is trying to use and Elder Scroll to cover the sun and let vamps run wild. The DLC adds conversations you can overhear about recruiting vampire hunters which then lead you the additional content. You can either join Harkon or the Dawnguard who are trying to stop him. Being a vampire gets you ability to become a flying vampire lord which work like being a werewolf and gets you a power called Vampiric Grip. This suspends enemies in the air while draining their health. You can also TURN INTO A CLOUD OF BATS! The downside to all this vamp power is that you will be attacked on sight although you can change back at any time. Joining the Dawnguard on the other hand gets you new weapons including the crossbow. Unlike bows these are always ready to fire and can stagger targets but take longer to reload. This faction also let you hire armored trolls as followers.
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Skyrim Dawnguard information