Months went by. The Dragonborn's hair grew longer and her skin more pale from being kept as a slave in the vampire castle's dungeon. Lord Harkon was quite cruel to her, and she began to welcome more and more Serana's more gentle sessions with her. The Mistress and slave grew closer emotionally, until one night Serana offered to make her an equal by giving her 'the dark gift' - the bite that would make her a vampire.
Given the offer of becoming Serana's lover on an equal basis, the Dragonborn didn't hesitate. She agreed to become a vampire. So Serana, as gently as possible, performed the deadly embrace.
Newly transformed into a vampire, her body was strengthened and enhanced. Her face was rather monstrous, but that would wear off with her first feed. No longer a slave, she was allowed to put clothes on.
And that's the end of the untold story. From here on, things progressed for her as told in Skyrim and Dawnguard.
