In my dream last night, there was a young girl with the ability to become invisible, although in the dream it was "Predator-invisible", which isn't quite undetectable. She was in a school/boarding house type place for magically gifted people, and the school was invaded by a single man with a sword. This was significant because there were supposed to be barriers around the school preventing people from getting in, so he must be powerful. He spoke of coming back with a full force and left. In the dream I actually pictured him getting a sword from Warcraft III, but of course for purposes of fiction I would probably have to change that to something else. Anyways, the last scene I dreamed was that some time later the invasion of the school had begun. There was fighting in the upper rooms. The girl was in the lower area and she watched a golden coffin descend from the ceiling (though not through the ceiling... the coffin simply warped in from nowhere) with a woman with a sword, who was one of the invaders.
As I thought about why the girl would be at the school, after I had woken up, I imagined that perhaps she had been abused by her father and had escaped to the school. She would have nightmares where her father was replaced by an ogre, coming up the stairs, she in her darkened room, feet block the light from under the door, it opens, she whimpers, the ogre reaches for her face and says her name, she screams, then realises that someone was saying her name and shaking her to wake her up from her nightmare.
As I was driving to work I imagined that the school might be called The House of the Four Pillars. Each pillar represents a faction within the school. It was originally the House of the Five Pillars, but one faction was exiled, and that is the faction which is invading the school. I imagine that the girl, I'll give her a code name of Emerald, is in training to join the helper/medic faction, called the Reborn. Before she can join, she is given a dagger and a test in which she confronts her father who is abusing her sister. To be Reborn she must resolve the situation peacefully, but ends up slitting his throat, which causes her to join the assassin faction (the Righteous?) instead.
What I would like to do is write the story from the standard hero perspective, but then show that her actions have been villianous all along. One angle to consider is that the school is actually evil and the helpful things they do are merely a front, and the exiled Pillar are actually the good guys. Perhaps Emerald will join them, but seeing them as the enemy she will eventually assassinate one of their important members.