Elsa
"Anna, it's time to give it up," said Elsa as she thumped on Anna's door. Anna hadn't come out since she'd left dinner the night before and Elsa was growing nervous. Elsa knew that Anna was a bit of a night owl and tended to sleep late, but it was well past ten thirty now and it was time that Anna accepted what was happening.
"You said noon!" cried Anna past the door, and a puff of black fog crept out from under the door. Elsa sigh and sent a rush of cold air to blow it away. It had been exciting the first time but now it was just frustrating.
"I said that he could have it back my noon, not that you could use it until then! Anna, set it down and come out and join Kristoff and I for breakfast. You've already been playing with that mirror for longer than you should have. Honestly, this is worse than the crown."
"The crown which you snatched away from me! This might be the only chance I'll have to play with magic without having to turn to you, so you'll understand if I want some time alone with it!" said Anna angrily.
Elsa stepped back, hurt by what Anna had said. Images of their childhood flashed through her head and she never once remembered Anna bearing this kind of jealousy towards her powers. "Fine. I'll go have breakfast with Kristoff. Just him and me, alone." She didn't know why she said that last part. It just popped out, but nevertheless she turned around and left without another word.
She heard Anna scream and the hall became pitch black, but Elsa kept walking. If Anna wanted to have an temper tantrum, that was her prerogative. The fact that her magic was as powerful as it was after she'd only had it for a few days was a worry that she did her best to quash.
Later
"So you just left her alone?" shouted Kristoff as they ate breakfast.
"What else was I supposed to do? Break into her room and take it by force?" asked Elsa as she piled bacon and eggs onto her plate.
"Yes!" exclaimed Kristoff, throwing his hands into the air as he did so. "That thing is a bad influence on her; I don't care how magical it makes her! In fact, that is exactly the reason that you should have just taken it from her! Ever since she's started playing with that damn thing she's been acting strange! It's like she's a different person now."
"She's just learning about her powers, that's all," said Elsa quickly. She'd spent thirteen years apart from her sister and didn't want to consider the idea that Anna could shift so into something alien to her nature in only a couple of days.
"Well, I'd say that if she keeps this up, then pretty soon her powers are going to change her into someone else."
Anna
"Anna, I told you already, it's time to come out and give it up. Lord Derek knows, I'm sure, so denying it won't do anything." Elsa's voice rang through the doorway like church bells and Anna was in hysterics.
"Mirror, I'm still haven't gotten a grip on mysting but I can't keep you any longer or Elsa will break down the door, I'm sure of it!" Anna said in a panic.
"You could just myst out with me and spend more time learning. That way the next time Elsa came up against you, it would be an easy matter to put her down," said the mirror in the same monotone it had used to speak to her with the whole time it had been with her.
"I don't want to put her down, I just want her to take me seriously!" Said Anna indignantly, even though a part of her indeed wanted to prove once and for all that she was better than Elsa.
"Well giving me up won't do that, your highness," the mirror said serenely.
"I know!" shouted Anna. She then concentrated and tried to myst across the room. According to the mirror, mysting would allow her to travel from place to place in an instant, but it seemed that all it let her do now was turn into a pillar of smoke and get her a headache.
"Anna, are you talking to someone in there?" asked Elsa through the door.
"No! I mean, um, no. It's just me and the mirror in here, that's all," said Anna, closing her eyes as she did so. She'd always been a terrible liar and she was sure that Elsa would pick up on it.
This time, though, Elsa either missed it or ignored it. "Alright, if you say so. I want you to know that Lord Derek is here now, and is waiting for the mirror. He knows you have it, Anna, and if you don't give it up I think that he'll just barge up and take it."
"What about his treasured peace?" Asked Anna through the door.
"I think that this is more important to him than that," said Elsa in a worried tone.
Just then Anna heard Lord Derek's voice in the hall. It was muffled so she didn't know what he was saying, but he didn't sound happy. "Mirror, what do I do?" she asked.
"Do you trust me?" it asked her, and she nodded as she heard an argument spring up in the hall. "Place your hand upon me, Anna," commanded the mirror and Anna did as she was told. "Concentrate on the North Mountain."
Elsa
"Lord Derek, those are Anna's private chambers! I forbid you from entering them!" Cried Elsa.
Lord Derek ignored her and pounded at the doors. "Your Highness! I told you once and I'll tell you again! Give it up!"
"You asked for it back and I said I would get it to you! That does not excuse your storming into our personal chambers!" said Elsa. She didn't want to freeze him, as freezing people in place was something she was still working on, but she wanted to do so now more than ever.
Lord Derek turned and glared at her. "Your Majesty, last night Princess Anna invade my mind using a magic that has only been used by the Witch Queen before her. I cannot believe that it is a coincidence any longer, considering the nature of the powers she used last night. Though I am loath admit it, Captain Wells might be right about the Anna's new nature. Jenkins, help me with this door."
Elsa was too stunned to stop him. Anna had invaded his mind? That might explain his headache, but it didn't explain her behavior, but by the time Elsa had moved to stop him from doing anything else, Lord Derek and Jenkins had already broken the lock and had raced into Anna's room.
"Princess Anna, the mirror, now!" shouted Lord Derek in anger.
Anna, meanwhile, had placed her hand upon the mirror in shock. Within moments a dark cloud surrounded her and she and the mirror vanished in a stream of smoke out the window.
"Damn!" shouted Lord Derek upon her disappearance. "Damn Damn and double Damn!"
"Are you happy now, Lord Derek?" asked Elsa as waves of cold emanated from her. "Anna has run away and I don't know what to do to bring her back."
Lord Derek looked up at her with a passionate fury she had never seen in anyone before. "No, your Majesty, I am not happy. Witnessing the birth of a witch should never be a happy occasion."
