Elsa swallowed thickly. Feeling all sorts of uncertainty and shyness, she shifted uncomfortably. It wasn't like Anna would ever purposefully hurt her; she had unwillfully hurt Anna once, but she didn't want her little sister to suffer the fate of blaming herself.
Anna breathed slowly, watching her monarch in the throes of indecision. Were it not for the inhuman thirst that had prompted this conversation, her elder would have had a stress free night of contented fulfilment. Unfortunately, her baser instincts forced her to confront Elsa about her own problem that left her craving someone to validate her very existence constantly.
The blonde looked up at the typically energetic girl, and she paused to think. There was no way around this conversation, and all she had ever wanted was to protect Anna. Innocent, joyful, carefree, Anna. The epitome of everything she had ever wanted.
"Anna, are you sure?" she started, mumbling to the floor.
The eyes of the other woman in the room narrowed, and darkened slits remained, framed by the endless sea green Elsa had always associated with single-minded happiness. The young girl leaned forward, and grinned. Her sharpened eye teeth glittered with purpose, and an oddly pink tongue showed itself.
"Elsa, if there was any other way, I'd have done it long ago."
The Queen gulped, and felt the spots below her ear. It wasn't like she didn't remember Anna's tender embrace; needy teeth had made scars appear where she could hide them from anyone. Anna's dependence on her concerned Elsa. Returning her gaze to the constant watchfulness of her sister's vision, she breathed deeply, and continued.
"It's not like this is the first time, Anna. I'm not really concerned that you'll hurt me."
"But?"
Shrugging the elder simply let her eyes fall on the fireplace, observing the patterns of flames licking at the firewood.
"But," Elsa continued, "couldn't you just, I don't know - feed off the staff?"
"Sister, you know that's not how it works. The curse Pabbie laid on me turned me into a vampire, and the only way to feed me is through magic. Your magic."
Leaning back and crossing her arms, the Crown Princess simply closed her eyes, and let the older sister think. Finally, she heard the woman sigh and shift. Her eyes snapped open, and raked down the figure of her sibling, standing before her like a supplicant. The bracelet that served as the Queen's reminder of who her owner was glittered on her wrist, and Anna stood to look her in the eyes.
"Fine," Elsa relented, "tell me what to do."
Smirking, the young redhead took a step forward, and put her arms around her most precious possession's waist, rubbing her nose against Elsa's. Kissing loosened her up, so Anna simply kissed her sister's lips, running a reassuring tongue across her lower lip, and watched her lover's now-peaceful gaze. Parting their lips slowly, Elsa bowed her head against Anna's and gulped air.
"It's nice that you don't need to feed that often."
The vampiric sister wrapped her arms around Elsa's torso, and kissed her earlobe, then let her tongue caress her favourite spot.
Anna's fangs dug in, and she propped Elsa's submissive form against her, as she continued to french kiss the wound made by the love that made her heart beat.
AN: I don't think trolls should be benign, and there being a horrible cost to saving Anna's life makes sense. Like having to feed off Elsa every 10 or so years. Also note, that Elsa lives as long as Anna; they are completely codependent in a good way.