Hogwart's Elven Student

Darian Griffin Quincey is approached by Magic after ten years of war after Lord Voldemort won against Harry. Magic sends him back in time, but lands him in a female elf's body, gives him/her a wand, and she manages to make herself Harry's blood-sister, swearing to protect Harry no matter what happens. How will old Voldie and Dumblefuck handle it? Enter gender-switch potion!