Ep. 23: Yukio confronts Ernst about the Gehenna Gate plot and his demonic heritage.

23

Yukio threatens Ernst after discovering plans to use Rin as a sacrifice to open the Gehenna Gate and deploy the Messiah weapon. When Yukio offers to take Rin's place, his blood mixes with his brother's, awakening his demonic powers. Satan contacts Yukio, revealing his mother Yuri's past as an Exorcist who survived his blue flames. She allowed Satan to possess her to experience Assiah, resulting in the birth of Rin and Yukio before Ernst ordered her execution. Satan caused the Blue Night massacre while seeking a new vessel, eventually using Ernst to free Yuri. Shiro and Mephisto were sent to kill them, but Shiro adopted the twins instead. Mephisto sealed Rin's powers and deceived the Vatican. Yukio confronts Ernst, who confirms Satan's account, and is accidentally pulled into the Gehenna Gate. Satan then possesses Yukio, recognizing their blood relation.