The world is about balance. Angels and Demons. Good and Bad. Holy and Evil. There are three different types of Angels, Archangels, Guiding Angels, and Guardian Angels. Likewise there are three different types of Demons, Demons of Death, Demons of Possession, and Demons of Sin.
Guardian Angels watch over humans, doing there best to make sure that little to no harm befalls them. Sometimes the Guardian Angels fail, sometimes they step back and give the person space.
Guiding Angels are responsible for leading a soul into Heaven. And trying to keep it from the Demons of Death.
Archangels are the defenders of Heaven. It is there job to watch the Gates of Heaven, and make sure that the Gates of Hell aren't opened.
Demons of Death are behind all violent deaths. When one dies, they attempt to drag the soul into Hell. Guiding Angels try to stop them.
Demons of Possession have the ability to posses some people. They are extremely powerful and extremely hard to get rid of.
Demons of Sin are mostly there whispering for one to do sin, Lust, Envy, Gluttony, Pride, Greed, Rage, Sloth, Murder, Abuse, and others.
This is how the world is, perfectly balanced.
Or so it was.
Until The Three were born.
Three Angels and Three Demons fell in love and each bore a child. The children are siblings in this sense. The mothers sent the children far away to a different time as they and there lover were all set to be tortured for the rest of eternity.
The Three that shall be born: a girl with an impenetrable mind, a boy of stealth, and another girl of cunning.
On The Eighteenth Year all three will be ripped from the life they known.
And then they will find each other.