Explanation
Episode 01 is the 1st episode of The Devil's Boy.
Plot
The episode starts right after the Prologue, where Jungho meets Mephistopheles for the first time.
The scene shifts to Lee Kyungjoon, who was still wondering how did he feel the pain from Jungho burning his tongue, and while he was still confused about what happened, he vowed to kill Jungho.
The scene shifts to Jungho and Mephistopheles, who explains the nature of Jungho's powers: Every time Jungho harms himself while thinking about a person he hates, said person will be the one to feel the pain, while Jungho won't feel anything. And the fact that it holds a price: With each time Jungho uses his powers, he loses an equal amount of his lifespan. This caused Jungho to grab Mephistopheles in his hands, telling him that it's not fair, only for Mephistopheles to tell Jungho that he knows that he wants this power in order to avenge himself from those who wronged him in his life. This eventually led Jungho to accept the deal, with Mephistopheles sealing the deal by branding Jungho with the Leviathan's Breath tattoo on his right arm, and before disappearing, Mephistopheles told him that his powers shall last for only 30 days, and that when those days are done, he will no longer exist and his soul will become Mephistopheles'.