On the wedding night a ritual which would bend them together.
The ritual would have them 'paint' runes onto which other's body (they become invisible to everyone once they begin their night) which were for: loyalty, love, and two others one for the mother called childbirth (which would help the mother get pregnant and give a save childbirth) while the father would have energy (which would give his dick more power for the baby making).
Draco used some magic on the Potters trying to find a way to get back at Harry which showed that the ritual was used that night was faulty, and during detention he freezes Lily before using some magic to connect the magic of the rune on her to himself which had Magic believe that he was the Man that she promised to marry all those years ago'.
Lily can still hear so Draco tells her that James had made a mistake which made her not a Potter at all, Lily quickly tried to resist but her mouth opened with no words. The ritual and runes itself, having already recognized her willingness to marry a wizard and having waited for almost a decade to do so, seemed to push her forward. The bond that Draco had forged took her initial agreement for James as proof of her acceptance of a husband, any husband.
As the magic of the ritual surged around them, Lily's eyes welled with tears, and she managed to choke out the words, her voice trembling, "I... I will."
And now she's has become Draco's and due to her vow, that was made with James in mind, no matter how much she desired to do otherwise she complied with all of orders Draco's.
