The orphan kid. Saturo hagukune walking in streets falls off a sewer. Inside the sewer he sees a bright light. He goes near the light he sees a man dying. He panics and says "Are You okay ?" the man says " who are you kid" the kid explains that he is a orphan who lives in streets but suddenly a demonic creature comes there who had already defeated the man. The man says "I can't Save you in the state I am so Inherit my powers and fight that demon" The man takes out a bright crystal from area of his heart and tells the kid to insert neare his chest he inserts it and feels power rushing inside him and he suddenly a bright green light comes from his heart and he transforms and grows a scythe from his hand and fights the demon and defeats it. Then he takes the man to his shattered house to cure his wounds.

After Saturo brings the man and cures his wounds the man tells him" In this world there are many other demons like that and they are trying to erase humanity so we core slayers stop them. I have my power to you so you have to slay those devils with your powers.

Saturo asks "But how can I do this" the man says "You have a green core crystal now if you train harder and kill core devil's the Core devils leave a crystal after dying you can increase your strength with the crystal. the green is the weakest and the bright white is the strongest crystal" you have to get powerful and try to save humanity I no longer have the power to do so you need to be my replacement".