Kratos and Anna, the two parents that bore the little boy, had to leave the Soul Society. Anna was experimented on by none other by the lieutenant of the 5th Division in the Gotei 13, Sosuke Aizen. Sosuke Aizen experimented on countless people such as Kensei Muguruma (the captain of the Ninth Division), Mashiro Kuna (lieutenant of the Ninth Division), Shinji Hirako (Captain of the Fifth Division), Hiyori Sarugaki ( the 12th Division's lieutenant), and some more. Aizen was conducting Shinigami/Hollow hybrid experiments back then and was the cause of the former captains and lieutenants disappear. Hollows was the enemy of Shinigami. They are lost souls that did not go to the Soul Society when they died. Hollows hunt and consume souls of human beings whether or not they are alive or dead. These monsters can cross over to the World of the Living or the Soul Society. Shinigami exist to help lost souls and Hollows rest in peace and cross over to the Soul Society. Sosuke Aizen believed that since he was a Shinigami Captain, he had reached the maximum strength he could get, but he did not want that... He wanted more power. He began conducting these Shinigami/Hollow hybrid tests to see if he could reach a power stronger than what he is now. The disappearance of the captains and lieutenants of the Gotei 13 happened a long time ago, but nobody in the Soul Society knows how or why they disappeared because the information was not discovered.
Anna, lieutenant of the First Division of the Gotei 13, was feeling weak while conducting some research.
"Ugh, Damn it... I'm feeling weak." Anna said while she almost fell.
"I shouldn't do so much work after I gave birth, but I have to know. Why the hell did the former captains and lieutenants disappear? It doesn't make sense... Captain-Commander Yamamato sure doesn't know..."
A voice came into her study, "Lieutenant Anna F- I mean Aurion.."
"It's fine... you can still call me by my other name." Anna said calmly to her subordinate.
"Captain Sosuke Aizen wants you in training ground behind the 5th Division's quarters!" said the subordinate.
"Captain Aizen? This late at night? Why..." Anna whispered.
Despite her suspicion, she started to walk to her destination. Anna was a talented Shinigami despite her age. She was promoted to the lieutenant position of the 1st Division led by Captain Commanders Genryuusai Yamamoto and Irelia Yamamoto, a prestigious position out of the whole Soul Society that any shinigami could obtain. She became friends with Irelia Yamamoto and that soon made Anna good friends with both Irelia and Genryuusai. She pondered her years in the Soul and she said to herself that she was proud of herself. Despite her mishaps in the past, her life was beginning to get better. There was no troubles bothering her, she married a husband that she loves and she had her first real son. At that point, it was like heaven for her... but not all lives are perfect... Before the night ends, she'll soon find that all of this happiness... was going to end soon...