Consider the Facts...
The Sōkyoku Hill overlooked most of the Seireitei. Sitting high atop the highest cliff in Soul Society, it was connected to the Senzaikyū, the palace of remorseful sin, which held the prisoners of Soul Society. A little over a decade ago, the hill was the home of the Sōkyoku; an immense blade that use to execute criminal Shinigami. On the edge of the immense cliff, Raian Getsueikirite and Genryūsai Shigekuni Yamamoto stood side by side staring out over the horizon.
"What did you wish to inquire of me, Getsueikirite Raian?" Yamamoto asked in his aged yet deep voice.
"The war is nearing its second day." Raian began, "As I'm well sure you're aware. I faced Akujin briefly, so I know this for sure... no person on either side currently has enough power to defeat him."
The ancient Captain-Commander stayed silent for a moment before speaking, "You are wise to recognize this. I too have come to that conclusion. That is why I believe this united spiritual force stands the best chance. No one person may be able to, but together, we stand a chance." he explained.
"Maybe so... but in the event that the army fails..." Raian began, though he was interrupted.
"Someone who has a chance should be present?" Yamamoto finished. "That I have also considered. However, not even among you Yonkō, does anyone exist that can fight him and survive."
"It's not like you to just give up like this, Yama-jii." Raian joked to break the tension.
"Foolish whelp!" Yamamoto comically roared in retort, "As the leader of the Gotei 13, I don't have the convenience of quitting! That's why I've come up with a plan."
Raian's eyes widened at this, "What is it?"
Yamamoto sighed, "We could join together. A 'union of souls', if you will." he replied silently.
"You mean, like Fusion?" Raian asked loudly, shocked by this answer. "Does such a thing even exist?"
"Silence!" Yamamoto bellowed, shutting Raian up. "It is a technique that was forbidden nine hundred years ago by the Central 46. For Soul Society's sake, however, I am willing to break the rules this once."
"But why me, Yama-jii?" Raian asked truthfully, "There are plenty of Shinigami stronger than I am. Kamui, Seireitou, and Echo among them."
"Good god, boy! Be silent!" Yamamoto snapped. "I will unite with you because you have the strongest heart."
Raian's eyes widened yet again. The tremendous weight of the words the ancient captain levied on him was overwhelming. It was a great honor to have such things said about oneself by Yamamoto.
"Kamui is too prideful. His heart could never contend with Akujin. Seireitou has wit, he has conviction, and honor. But his heart isn't in this like yours. You were the one to defeat Akujin in the last Crisis. You had your brother torn away from you by this man of evil. Only your heart could ever defeat Akujin's hatred." the aged captain continued.
Raian bowed his head and remained silent. He was bound by a sense of modesty not to agree with Yamamoto, but in his heart, he knew it was true.
"And only you," Yamamoto concluded. "Could ever hope to lead this army. That's why tomorrow, they will vote on you to lead the new Spiritual World Army. The Espada have been swayed to join us by your courage during the last war. The Humans of MisQ respect your love and compassion because of your time spent with them after the Central 46 exiled you following the war."
Yamamoto slammed his staff down and raised his baritone voice to denote the seriousness of his message, "And I speak for the Gotei 13 and Shinigami of Soul Society when I say, we respect your heart. The heart you showed when Mashū Getsueikirite needed you and you protected him. The heart you showed when Akujin's men took his life and so, in holy righteous fury, you avenged your comrade while protecting the numerous other comrades that were present for your battle. Your courage swayed the Hollows, your love swayed the Humans, and your heart swayed we ancient beings of Soul Society! That's why I will join with you."
Raian could no longer hold back the tears that poured now from his eyes. He had failed to recognize it. For decades, he had impacted the lives of many people. During Akujin's initial invasion, he had allied with renegade Espada to fight Akujin. During his exile in the Human world, he had helped and protected the Humans of MisQ. And even though they initially had outcast him, he had always been a fierce ally of Soul Society. Throughout his battles and his achievements, he had failed to notice how many lives he had impacted and how many people had changed because of him.
"Dry your tears, Getsueikirite Raian!" Yamamoto roared. "I will explain to you the mechanisms of what the Fusion consists of. But first, I must tell you about one thousand years ago. Long before a whelp like you was born. I must explain to you the real reason behind Akujin's actions in this war. His real plan!"
Yamamoto bowed his head and began his lengthy explanation, "One thousand years ago, the Gotei 13 didn't exist. Shinigami were freelancers back then. They did whatever each individual one wanted to balance the world. It wasn't too long, however, that two factions arose within the Shinigami universe. I headed one. The other group, called the Getsueikirite, was headed by a Shukai user, and the one that got the name Shinigami applied to us. Above even us, however, were two spiritual beings that ruled over all; the Spirit King and Lucifer, a spiritual being unlike any you've ever met." he explained.
"A short time thereafter, Lucifer decided that he wanted to rule all three planes of existence with no one to equal him. He staged a rebellion by freeing the Togabito of Hell and leading an attack on Soul Society. In response to this, I formed from my group of Shinigami, the early framework of the Gotei 13 in order to combat Lucifer's evil. The Getsueikirite, however, remained on their own. The war with Lucifer cost hundreds of thousands of lives. When all seemed lost, the leader of the Getsueikirite activated Shukai and, with the tremendous power it granted him, not only defeated Lucifer, but sealed his immense power into three cores. We call them, the 'heart of hearts'. They lie at the core of each world. It is my thought that Akujin is attempting to break these cores in order to free Lucifer once again."
Raian stood in utter horror as he absorbed all this information. It was as he thought in the Human World; Akujin really was after something far more sinister than just dimensional domination. He was going to revive evil incarnate to accomplish his goals.
"There is more you need to know." Yamamoto said, his voice suddenly much more solemn. "Akujin is merely a title. A moniker, if you will. You see, only two lines can be traced down from the founder of the Getsueikirite's family. The line that spawned you and your brothers and the the line that spawned Tadoku Getsueikirite. Akujin is your uncle; Tadoku Getsueikirite!"
Raian was stunned beyond belief at this. His parents, when he was young, had never mentioned an uncle. Now he knew why. Akujin - this, Tadoku Getsueikirite - was a man of unspeakable evil; a shame to the Getsueikirite Family. Now it was Getsueikirite vs Getsueikirite. He silently wondered if Seireitou had known of this development.
"Do not be shaken, Getsueikirite Raian. I have said enough. The rest of it, you will know soon enough. It is time I explain the mechanics of our union to you. The time has come." Yamamoto declared.
The Union of Souls
Raian sighed, having finally taken in the immense amount of information he had received. He then nodded at Yamamoto.
"Alright, tell me how we will do this." he said, determination returning to his eyes.
"Listen carefully, I'll only be able to tell you this once." Yamamoto said sternly, "In order to do this properly, we need to use you as the base of our fusion. This means that your body will remain the same and your conscious will remain dominant. Our powers will merge completely at the most basic level. That means that once we are fused, there is no way possible to undo it. You will also obtain my mind, my knowledge, and my wisdom."
"What will happen to you?" Raian asked, concern showing on his face.
"Do not look upon me that way, Getsueikirite Raian." Yamamoto snapped, "It is time I entrusted the future of Soul Society to the younger generation. But, if you must know, my conscious will live on within the Inner World of your soul."
Raian sighed in relief, "Very well. I am ready. What should I do?"
"Place your hand on my chest and I will do the rest." Yamamoto instructed.
Raian did as he was told. At first, he hesitated, the weight of the decision they were making finally dawning on him. After a moment of thinking on it, he continued, placing his hand firmly on the ancient captain's chest. He then nodded to his aged comrade to begin.
"Getsueikirite Raian! I bequeath to you the future of Soul Society!" Yamamoto roared, "From the calling, two stars shine brightly! Coexisting! Standing firmly, propelled by their own hubris, pointed in the direction of desire! Moving forward! Thrusting forward toward the nexus of all nature! Unite! These two shining orders, coexisting, moving, thrusting, uniting! Fuse!"
Spiritual energy erupted from the hill, blasting its way into the sky and obliterating any clouds that were anywhere remotely near the Seireitei. A tremendously dense spiritual pressure fell on Soul Society, creating a small earthquake to shake the entire plane of existence. When it finally dissipated, Raian stood among the ruins of the Sōkyoku Hill, cloaked in Yamamoto's immense captain's haori; a new sword gripped tightly in his hands. As he looked up from the rubble, an intense look of fiery passion shone bright as the sun in his eyes.
"Let us begin...!" he snarled, and erupted from his spot heading directly for the Seireitei.