23화: 유키오가 게헨나 게이트 음모와 자신의 악마 혈통에 대해 에른스트를 마주한다.

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Yukio threatened to kill Ernst after learning about his plans to open the Gehenna Gate with Rin as a living sacrifice and to drop the Messiah, a weapon of mass destruction, into the Gehenna Gate in order to completely destroy Gehenna. When Yukio offered to take his brother's place instead, his blood mixed with his brother's blood, which awakened his demonic powers. After the Gehenna Gate, Satan makes contact with Yukio, showing him the past of his mother Yuri. She was an Exorcist who was sent to investigate mysterious spontaneous combustions, only to encounter Satan, but she was immune to his blue flames and survived his attack, later searching him out and letting him possess her so he could experience Assiah. After sharing her body for over a year, she ended up pregnant with Rin and Yukio, but her own father Ernst ordered her burnt at the stake. Satan then unintentionally caused the Blue Night massacre as he searched for another vessel to save her. Eventually, he used Ernst as a temporary vessel to free Yuri, who fled back to the woods and gave birth before passing. Mephisto and Shiro were sent to hunt down and kill Yuri and her children, but after witnessing Yuri's last moments with her children, Shiro was unable to bring himself to kill the twins and adopted them instead. Mephisto decided to help by sealing away Rin's powers and lying to the Vatican about completing the mission. Yukio confronts Ernst, who confirms Satan's story, and inadvertently causes the man to be sucked into the Gehenna Gate. Satan possesses Yukio as his vessel, realizing they are related by blood. (Source: Wikipedia)