Dawnguard (quest)

Walkthrough
Hear of the Dawnguard vampire hunters and the locate Fort Dawnguard.

Speak with the leader of the Dawnguard
Speak to their leader, Isran and join the Dawnguard.

Note: Even though the Vampires vs Dawnguard quest line starts with talking to Isran, it is only much later, after returning Serana to her castle home, that the Dragonborn actually needs to decide what faction to join. Dämmerwacht (Aufgabe)