I think this one's kind of bad. Oh well...
Elsa was never born with Ice powers. She and Anna had a beloved book of fairy tales and would sometimes pretend to be their favorite characters from the stories. Anna would pretend to be the long haired magical princess of Corona. Elsa pretended she had as powers, like The Snow Queen.
When Anna was 5 and Elsa was 8, they were playing that very same princess game, when an avalanche struck. They both were buried under the snow, and Anna was killed instantly. Elsa on the other hand was required in time, but suffered a severe blow to the head that damaged her brain.
Fantasy blended into reality for Elsa and she began to believe in the imaginary game she and Anna used to play. She did not believe that Anna was dead, and she would speak to air sometimes, as if it was her dead sister. She thought that she had hurt Anna, as a result of her having Ice powers.
The King and Queen of Arendelle sot out doctors, but there was no cure for Elsa. To hid her insanity the closed the gates, reduced staff and kept Elsa in her room at all times. They kept telling Elsa to Conceal it, but she thought that they were referring to her Ice powers.
Elsa condition only got worse and she believed more and more in her delusions. Eventually she only would talk to her apparitions and believed her parents to be dead, since she could not see many real people anymore.
She was still aware that she was royalty and on her 21st birthday, she began to imagine her coronation. Her parents could not allow Elsa to be actually crowned Queen, but Elsa went around talking to invisible people, and acting out the coronation ceremony. All went wrong however when Elsa escaped the Castle, under the impression that her powers had been found out.
The King and Queen sent out search parties, but no one could find her. Eventually all hope was lost and the search was abandoned.
2 months later, a man came running down the North mountain, carrying a dead girl and his dog. He was cut and had a crazy look in his eye. Kristoff(as he was called) was hiking up the North Mountain with his girlfriend Annie. They went exploring in a cave and found a crazy girl called Elsa. She claimed that the cave she lived in was an Ice palace and that she had ice powers.
Once she saw Annie, she brightened up and kept calling her Anna, and was under the impression that she was her sister. She called their dog Olaf and she wouldn't let them leave. After a while, Annie got fed up with the crazy girl and shouted that she wasn't Anna, and that she had never seen Elsa before in her life. Elsa, then became quiet and sinister. She said that since she wasn't Anna she couldn't let her leave because know one could know where she was. Kristoff and Annie tried to escape, with their dog, but Annie was killed in the process.
Kristoff said Elsa's last words were: I'll find you Anna, don't worry. And then we can live together... forever