Dawnguard

You reach for the image of the Dawnguard's crest, remembering the route you took after rejecting Harkon's gift. You remember how Serana left the castle in search of you. She trusted you. The both of you fought side by side. You trusted her. As a team, you both saved Dexion. Dexion read Serana's Elder Scroll aloud in front of you two. Serana and you slipped into the abandoned part of Castle Volkihar in search for Valerica, hoping that she head another Elder Scroll. Then you found the portal to the Soul Cairn. As a mere mortal, you were not able to pass through the portal without dying. Serana gave you two options.

As a vampire, Serana wasn't counted among the living, and could pass through the portal without sacrificing her soul. She could turn you into a vampire so you could do so as well.

Or, if you didn't want to become a creature of the night, she could try partially trapping your soul into a black soul gem, and offer that as sacrifice.

You remember what you told her.