The prologue to the story:
It started with an evil goddess who created elf like beings (tall ones) for her amusement. Some time after being having enough, the elves overthrew the goddess with all their power united. As the connection between the elves and the evil goddess severed, the goddess left behind an evil entity (it looks like a long leech-y creature) to torment the elves.
Having no solution how to kill it, the only option the elves had was to seal the being into a host body. It pained them greatly to do such thing, but they had no choice as the entity posed too much threat. When no one would ever volunteer to be the host body, a child elf walked forward, signaling that he would sacrifice himself to be the one to carry the entity. At first the adults were baffled why a child should be under such burden. As they talked to themselves, one of them was sparked with courage to be the host instead of the child. But there was the elves who lost their family member to the evil entity who wanted nothing more than to get the creature to never hurt anyone again. Those elves were blinded by their sadness that they, without telling anyone, went ahead to seal the entity into the child's body. In the process of sealing, the other elves noticed too late to stop them. To interrupt the sealing could only bring terrible consequences.
It was done.