The Dawnguard are a group of vampire hunters living and operating in Fort Dawnguard. You have to install the Add-On "The Elder Scrolls V: Skyrim Dawnguard" to join them.
You can join either them, or the Vampire Faction. The main quest line consists of twelve quests with the aim to kill the Vampire Lord.
Joining the Dawnguard has following Benefits:
- Master Level trainers in Archery, Heavy Armor, Restoration and Smithing
- Ability to craft crossbows
- Hiring armored trolls
- Radiant quests
To join the Dawnguard you have to talk to Isran at Fort Dawnguard.