Long ago there was a spirit with immense powers that was able to cure any disease or ailment and, though the spirit never used this ability, drain the life energy from someone. This life energy would make him more powerful for some time afterwards. If the spirit drew enough energy, it would eventually kill them.
[...]
One day a human king learned of the being's powers, and asked his friend - a powerful fae sorcerer to create a spell that would bind the powers of this being so that he may use it. The sorcerer tore the soul from it's origional etheral form, and placed it inside of a necklace. But the soul was irreparably damaged by this action, and was no longer the pure selfless being it had been previously. All it's emotions were taken from it, leaving only a burning desire for energy.
[...]
The king began to use the power of the soul, stored inside the necklace, to heal his people of dangerous and incurable illnessses, and mortally wounded soilders in battle. But over time, the king grew greedy. After defeating a neighbouring country in a war, that the other country initiated, and claiming their land of this country as his own, he grew to desire more. He invaded several human countries.
But on the eve before his battle with one of the few reamaining human kingdoms, a group of knights, were sent by their desperate king to visit the powerful spirits that lurked inside the magical sea side cavern, and they begged the spirits for their help in stopping the king from continuing his mad rampage of the human kingdoms.
The spirits gave each of the seven knights, a powerful ability, and told them that they must locate the necklace, and then use a powerful spell to unbind the spirits soul from the necklace, doing so would weaken it greatly, then, when it was at it's weakest, they should place the spirit into a living entitity and kill the being.
[...]
They progressed with the plan, having taken with them a young baby from the villiage as a sacrifice, but it didn't work, the soul could not be bound to the young child, for the child's soul was already too powerful for it to grasp onto, in it's weakened state.
But it was too late to find another sacrifice, and they weren't sure at the time why it hadn't worked. The magic circle broke, and the spirit, no longer contained within it, fled. It was now free to move from inanimate object to inanimate object as it saw fit, it could also move freely of these objects, though in that form it was incapable of using any of its powers or influence.
[..]
The knights went back to the spirits, to ask about their failure, and the spirits explained that a young baby was far to pure a soul to be controlled in such a way, and that they would need to
find a young foetus, still inside of the mother, and in the earliest stages of development, to make it work?
They continued their search for the soul, they had several near misses over the period of 1052 years, the soul remained ellusive, but in the year 1990, they found the artifact, the soul could of course move, so they cast a spell which would stop it from being able to move, for a limited ammount of time, and then, quickly carried out the ritual needed to bind the spell to the woman.
[...]
The soul had currently been occupying a bracelet, though it did not always occupy jewlery, it seemed to prefer to, and often did, because of the more person touch. People would often wear the jewelry making them more psychologically close to the soul, which was what the soul wanted.
He had found the bracelet, on the hand of a serial killer, who had been using it's powers to kill, whenever he got the urge.
[...]
After the soul was bound to the body of the girl, it began to think for itself and was capable of more than a simple lust for power. It developed a personality, occasionally slightly influenced by the girl's, but still noticeably different.
At some point, not long before the beginning of the story, the soul realised that it was capable of gaining some control, when she was in pain - either physical, or when she began to lose control emotionally. Though it was not possible to possess her completly, it was able to control the powers it possessed, and it manifested itself by sudden outbursts of it's power that she was incapable of controlling.
She began to think of the strange power as her own, but didn't tell anyone about it, a fact that annoyed the soul, as he disliked her laying claim to his power.