Unexpected Love in the Future

There is a man that is the most honored demon in the realm of the underworld he has the black diamond that is only one in a hundred years appear . While there is an assassin that kill her alcoholic father and her irresponsible mother, well that is not her real parent and her parent die in a car accident and the girl only survive because she is an experiment of the two people that die in the accident that's why she didn't die in the car accident that is supposed to kill her, and her parent because her parent didn't want to kill her and the scientist (parent that she knew)

Escape the organization and at that time she already meet the young demon who ones visit the human world and she make a promise to her that she's going to marry him in the future .

In the murder case of the two people who she kill ,she was taken by the woman in mask and a man with many scar in his face and body who dress in black she was taken in the organization of the Black Riven where she grow up and she moved out when she was 18 year old and the organization organized a house for her and she was always given an order by the organization where she kill and drink alcohol to forget many things like the promise and the scene where she kill the two person who abuse her by beating her
And on the way to her house she meet a young man in the alley and she see him killing all the people and she see the boy face and a flash back of the promise come in her mind and he look exactly like the teenage boy that she make a promise