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Once, it was two children, a girl and a boy that both was adopted. They was raised by a murderer, wanting some help with missions. They was both trained in years, separately. No of them having any contact with another human than their "father".

However, as Elizabeth, the girl, turned 16, she begun to think about life. Why did she have to kill people? Was this real life? Is this how everyone lives?

Deciding to want to explore the world, she fled and Meliodas, the 16 years old boy, was ordered to track her down and kill her. It didn't turn out that way though as she convinced and gave him a taste of freedom. They was slowly teaming up and no one knows how it will turn out to be.