To all the people that is confused
(ctto btw)
They used Gyeong’s soul to heal the wound in Cain’s heart from the bullet.
When Cain is wounded he subconsciously absorbs all the power surrounding him to heal himself without realizing.
Because Gyeong is “human” the amount of power that Cain is absorbing from his shouldn’t be possible.
Cain absorbed so much power from Gyeong that he became his full demon form, even bigger than when we first met him, even bigger than when he was fighting Satan.
They had sex because it was how they would open the door. They basically kinda were opening a portal so that when Cain was healed they could leave but Cain is a horny boy and ripped Gyeongs ass.
Anyway, when Gyeong started to disappear from his and Cains sex conscious, in the real world, Aaron (the doctor) and his little side kick saw the light of heaven come from the portal that Gyeong and Cain were coming out of
(Cain forced himself to come out with Gyeong because he didn’t want to leave Gyeong alone)
So from what I gathered was the doctor had a clone of himself and his head got chopped off or it was really him that got his head chopped off but he can come back to life but is the second hand of Cain that did it. Why? He said he was joining Satan's team but there was someone else outside in the scene. So when the second hand guys power came out. It was because he senses an enemy and Cain thought it was aimed for him. It's all so confusing. I won't be shocked if I am wrong. I don't think his trusted second hand man really betrayed him. I feel like there is something deeper going on.