I got an idea, these creature from another world came to the planet and had children with human's. These creatures, lets call them. Alters. The children these creatures create are called half-breeds, 70 years after this event the story takes place.
The story starts off with two half-breeds who parents were rich and leaves them in the mansion in the city. They managed to keep their abilities hidden to not cause trouble for themselves. How on the start of the new school year a half-breed girl joins his class. Due to certain circumstances he ends up reveling his ability to her and the two decide to try to track down the strange events that have been happening in the city recently that have been causing a wave of bad luck to go through the city.
The story can get dark and graphic, but that more due to the abilities rather then the ideas presented in it.
Alters are easier to tell, it usually comes down to eye color. Normal Human's don't have red or purple eye colors. Not that it matters too much in the context of this plot, since the sibling parents are away at the time. And the parents are rather good at keeping it a secret,
Just and outline, figuring out characters abilities and events that are written and what not.