Board Thread:General Discussion/@comment-11932638-20140818210026/@comment-67.166.65.251-20140824015731

Spoiler: Don't read if you don't want to be spoiled. I won't include that much but it's still a spoiler.

If you join the dawnguard, there will be a chance later down the road where you get to pick one of two options to progress the DG Storyline. Serena will offer to turn you into a vampire or continue as human with some stat reduction while in the next area. If you choose vampire, you get the perks and can finish the questline. However, most DG non storyline quests tend to get locked until you get cured so make sure you finish the crossbow quests first.