Rose Briarhoof was a unicorn born in the times when necromancy could get oneself executed, she was only 4 years old when her parents were accused of committing the crime, her mother sending her off with only the necklace she used to wear herself. Rose ran far off into the forest as her parents were trapped in their own home, the villagers setting fire to the house and killing her parents inside. for two nights Rose hid in a cave in the forest until a group of vampires found her, shivering and nearly starved to death, taking pity on her, they took her back to their castle and raised her as one of their own.
On her 18th birthday, Rose completed the ceremony to become a vampire herself, soon growing a new set of bat-like wings and fangs. She was out in the courtyard when the attack happened, Vampire hunters had seized the place, killing off all of her adopted family members, she was the only one to escape that night with her life and that particular subject is a bad one to bring up around her.
For many years Rose traveled alone, her only possession being the stoneless necklace her mother gave her on that night. She spent her time inspecting it and trying to think of her parents who she hardly remembered anymore. One day she turned it over in her hooves to find that on the back metal of the empty socket there was a scratched in picture of a hydra with a scar across one of its faces. Rose made it her goal to find the hydra and hopefully along with it, find the stone that used to fit in the necklace socket.
Over a span of 3 years she finally came across the den of a hydra, noticing that it did indeed have a nasty scar slashed across the middle heads face. After a long tiresome battle the Hydra finally fell, gallons of its crimson blood pouring from the jaws of its heads. Now, hydra blood has a strange effect on vampires that causes them to act like they're in heat... x2. Roses mind was a little too effected by it and has ever since been almost constantly perverse, she can't help it.
After the blood had dried and Rose had stopped trying to flirt with a boulder, she found a clear red glowing stone at the foot of one of the heads. Picking it up, she found that it was the exact size and shape as the socket in the necklace. As she brought the stone to the necklace around her neck, it had a magnetic effect and slipped permanently into the socket with a large flash of red light. Before Rose even knew what was going on, her body started taking on a new change, her ears twisting and forming into two smooth white horns, her tail becoming more draconic and her back growing another set of bat wings behind her first set. She was much taller now and a glowing red rage had completely blinded her eyes.
Rose doesn't remember much after that, only that she woke up in a large crater back to her normal self and around a third of Everfree was burnt to a crisp around her. She spent many years learning to control her transformation since the necklace was now forged permanently into her skin as if it was a part of her now. After a long time she finally became able to change at will in full control of her actions.
Rose currently resides in a tower on the outskirts of Ponyville near the Everfree Forest and helps with keeping bandits out of the town, everyone there doesn't seem to mind her presence and she's even made some good friends, for once Rose has finally found peace in her 903 years of life and she's content with keeping it that way from now on... with an occasional lingerie party every now and then of course.