Only one moment of abstraction, that was all it needed for the demon to sneak into the monastery.

Astaroth, the Demon King of Rot, made his way into the nursery to search for the newborn Prince of Gehenna; and there he was, a small pink creature with few raven hair sleeping next to another child with a few moles. He just didn't understand how humans could find those little flesh bags cute or precious.

Carefully, he picked the demonic infant up. "I will take you home, little brother," he muttered to himself. Even if it was weak and almost completely sealed away, he could sense the demonic origin of the baby.

Astaroth turned around and walked out of the nursery leaving the second child alone.

He was looking for the cursed sword, the Paladin used to seal away the Prince's flames. Suddenly he heard loud noise which woke the baby prince who immediately began to scream and cry. "Put the baby down.", the old man who just shot at the Demon King growled.

"Too bad, Lord Satan will not be pleased if I just bring back the little prince but not his demonic heart." With these words Astaroth made his way out of a window, careful to shield the newborn from the glass. He landed gracefully on the ground, the priest ran to the window but didn't dare to shoot another bullet because he couldn't risk hitting the infant. Astaroth managed to run into a deep nearby forest, thankfully the young Prince ceased wailing. The Demon King took out a crystal flask with it on the ground he began to chant. A monstrous gateway emerged where the flask was baby had curious eyes as he stared at Astaroth, attempting to touch his face with his tiny hands. Just before he could touch him, Astaroth put the child on the gate, allowing him to get slowly sucked into the demon realm. After the infant vanished, he left the host's body returning to Gehenna.

Shortly after, the paladin, Shiro Fujimoto, arrived. He could only witness how the Gehenna Gate disappeared, taking his adoptive son with him. He fell on his knees. Why did he leave the twins alone in the nursery? If he had been there, the demon wouldn't have abducted Rin. At least they didn't manage to get to the Kurikara, so the boy would be human and maybe, just maybe, they would return him, because he was useless to them with his powers sealed away and Father Fujimoto would make sure that no demon would ever lay hand on the koma sword.

"Lord Satan," He paused, "Father, I brought back your lost son." Astaroth proclaimed, kneeling down in front of Satan's throne and holding Rin in his arms. "Where is the koma sword? Without it, my son is merely human." Satan didn't sound to happy about the missing sword.

"I'm really sorry, father, but I wasn't able to find Kurikara before the old bastard discovered me. In order to save the young prince, I had to abandon his demon heart." Explained the King of Rot. Satan nodded in understanding; while normally he would have set Astaroth ablaze, but he wanted to see his youngest son (He's excluding Yukio, because he didn't inherit Satan's flames). "Bring him to me." The king of Gehenna ordered. Astaroth came closer to the throne presented Rin to his father. The baby was asleep again, Satan was sceptical of the boy and gave a sign to a bystanding servant. "Take your prince to his room. You will take care for him from now on until he is old enough to start his training. I've got big plans for him."