Another somewhat old project, originally conceived as a writing exercise to improve my somewhat lackluster battle scenes.

Have since put some thought into it and hope to turn this into the first of a series of short fics (one-shots, mostly) that follow a shared plot.

Invasion of Seireitei I

Juushiro Ukitake vs Aaroniero Arruruerie

Part 1 - The new old pain.

I- Stranger

Seiretei had been invaded. Again. And, this time, it was not a ragtag group of misfits in a desperate attempt to rescue a friend.

Jushiro Ukitake had seen many terrible things in his dozens of centuries of service to Soul Society, but it seemed that this night was blackest than any other he remembered... Pyrrhic victory or utter defeat, those were the only possible scenarios now, since abandoning Seiretei to Aizen's creatures was unthinkable for a great many reasons.

How the defenses of the fortress had been breached (again) and how the invaders had penetrated so deeply before being detected were still unknowns. And, for the time being, less important than the ongoing combats for these invaders were monsters created with the sole purpose of destroying the enemies of their master. Already parts of the citadel were burning and the very air vibrated with the release of massive amounts of spiritual energy.

It seemed that some captains had already joined the fighting and even released their swords. It... didn't seem to be doing much good so far. Some of the large concentrations of tainted spirit energy that he could detect with his well honed senses had seemingly accepted this challenge and raised their own power levels, some matching and some even dwarfing his peers. He itched to run to their aid, but dared not abandon his position.

Vicecaptain Sasakibe, bless his soul, had been handed overall command of the thirteen divisions by the Captain Commander, when Yama-jiji had decided to personally enter the battle, and being his usual useful self, Sasakibe had quickly identified a number of key locations that might be the targets of the invaders and arranged for their defense with the forces that could be spared from the frontline.

And so it had been determined that Captain Ukitake was to protect a nondescript building within the complex of the twelfth division that contained some rather important electronic machinery. The computers stored the spiritual characteristics of a number of very dangerous individuals and used this information to create and maintain barriers of unmatched power and density that kept these selfsame individuals out of Soul Society.

Things were hairy enough already, without Aizen and his chief accomplices getting involved in the fighting.

Mayuri Kurotsuchi was also supposed to be here to help protect the facility, but the madman had apparently decided that the call of science took precedence over mere military needs and abandoned the premises along with his vicecaptain, just before Ukitake's arrival. No doubt, captain Kurotsuchi was more interested in collecting living Arrancar specimens to study and have a chance to outstrip Urahara. All in all, however, the absence might be for the better and, at least, the insane scientist hadn't taken with him any of his underlings to use as bait. Probably Unohana-sempai's rather incensed reaction after hearing a story of shinigami turned into living bombs by their commanding officer had something to do with that.

Much like Shunsui or Yama-jiji, the captain of the twelfth had a bankai dangerous to both friend and foe. And while the seated officers of the research and development division weren't exactly powerhouses nor all that sane by most standards, they had been thoroughly cowed into obeying orders quickly, efficiently and to the letter. After he had... persuaded them to disregard the orders of their commander to ready the autopsy rooms, they had been quick to prepare for defense.

A particularly helpful young fellow called Nomura had even pointed that some of the experiments considered failures might be of some use in securing the facility. The head of the Shinigami Research Institute was a prideful man and the inventions that didn't satisfy his exalted standards simply weren't allowed to leave his domain, lest their very existence tarnish the reputation of his genius. Ukitake had known that much.

What he hadn't known was just how exalted those standards really were. Nomura and a few other members of the twelfth had led him to a vast storehouse behind the main lab complex and revealed a startling array of vicious inventions that for one reason or another had been discarded. A gas designed to inhibit the regenerative abilities of Gillians? A breed of red hell butterflies capable of storing kido spells of up to level fourty and tracking their prey? An armor made of a synthetic material with enough spiritual density to withstand the blows of a bankai?

There wasn't nearly enough time to get ready for use even one tenth of the contents, but...

A sudden burst of tainted reiatsu -close and as powerful as one of the younger captains- appeared in Ukitake's mind's eye, revealing that there was no time left at all. It wasn't nearly powerful enough to weaken him, but it forced him to raise his own spirit pressure. This, combined with the intruder's own presence, was enough to floor Nomura and the others. Whether they were unconscious or simply too weak to move when crushed by the power of an enemy of this stature was unimportant. Either way, they were in no condition to offer any help.

No matter. The white haired shinigami launched himself towards the location of his enemy at the speed of flash. After crossing the threshold of the storehouse, he located his target and readied his sword for the first blow. Though he crossed the distance between them both in less than a heartbeat, the battle instincts developed over many lifetimes of combat started to pick apart his opponent looking for some weakness.

Fairly human looking and human sized, but with a strange, elongated mask hiding both face and head. According to Urahara, shinigamification tended to leave deformities from the original hollow form in the lesser types of Menos. That meant that he was probably facing a former Adjuchas, although there was a possibility of the creature being an unusually strong Gillian. If it was the later, then the Arrancar might have regeneration. Ukitake decided to try to cut a shallow gash across the chest of his foe with the first blow, to gauge the density of the hierro and ascertain whether the enemy had the ability. An instant before contact, the captain filled his blade with his spirit energy, increasing the sharpness of the weapon and...

... failed to land a blow.

In the last possible moment, the arrancar whirled to the right like a dervish, dodging the attack entirely. The captain recovered quickly and continued his attack, but he had lost his momentum and the white garbed creature used its own sword to parry the next strikes. And then, before he could decide a new tactic, the Arrancar raised his hand.

"Bakudo 56: Yamatate," it said. And a barrier of golden energy appeared between both combatants.

Ukitake stopped. Not because of the unexpected kido shield (which he could tear down by punching hard), but because of the voice. He recognized it. And he recognized the smiling face that was revealed when his enemy removed his strange mask. Old pain, never truly healed and great, impossible hope blossomed anew in his heart.

"Whoa! You are in rare form tonight, sir! Glad to see your health has improved!"

"Kaien!?"

II- Imposter

The first thought that crossed Ukitake's mind was to drop his weapon and hug his old friend, returned from the grave. The second thought was to kick himself in the ass for being foolish enough to contemplate such a thing, even in passing.

Ultimately, Aizen's arrancars were just transformed hollows, empty spirits that had been consumed by grief and loss. And seeing that many hollows were adept in causing or exploiting such emotional states to weaken or cripple foes or prey, through use of shapeshifting, illusion or other tricks, it was only logical to assume that what he faced was exactly that. Ukitake himself had encountered many such things in his years of service.

Which meant that once again an unclean spirit was desecrating Kaien's memory. Unforgivable.

The captain's face hardened and he slashed with his sword, cutting through the barrier of the Mountain Shield as if it were gauze. The spell exploded into a cloud of glowing motes and the soul cutter continued towards the arrancar. However, before he could land his blow, the figure in white vanished in a burst of speed, using the arrancar shunpo knockoff to retreat.

Jushiro followed. He now had a grasp of his opponent in terms of speed, as well as raw power.

Aizen's minion was quite fast and his reactions sharp.

But Ukitake was faster.

They returned to the realm of normal speed at the same time, the shinigami already launching a new attack, the arrancar already preparing his defense.

A new Yamatate appeared in the path of the captain's stab, but it was a crude thing, hastily raised and swiftly penetrated. All the same, it marginally delayed the attack, allowing the arrancar to deflect the soul cutter with his own blade. Instead of the chest, where Ukitake had hoped to pierce the hollow's hakusui, he delivered a glancing blow to his left shoulder instead. The white fabric of his uniform was cut and, after sparking briefly, so wasthe hierro.

The false Kaien hissed in pain and once again retreated. This time, the shinigami captain let him get away and allowed himself to feel a small measure of grim satisfaction.

It had been a shallow wound. Very shallow. But first blood had been his and it was now clear that he had this foe almost hopelessly outmatched.

Victory would be his. And he would avenge Kaien.

He could never hope to compensate his failure from that night, but even a small catharsis would be most wel...

"It is really me, sir! So, please, don't cut me again. This hurts!" shouted the arrancar from his new position down the street, interrupting Jushiro's thoughts.

The captain offered no reply and merely charged again. And, once again, the false Kaien whirled out of the way of the first attack and managed to hold his own against the following strikes. There was an elegance in his swordsmanship that truly reminded Ukitake of his late friend's dance-like fighting style. It really was almost as amazing as it was sickening how very well Aizen had trained his creature.

He ignored the increasingly frantic words of his opponent and kept looking for an opening.

He found it when one of the hollow's whirling dodges left him close enough to the shinigami captain for a kick. So Ukitake kicked.

Now, Jushiro Ukitake was not and had never tried to be a great master of unarmed combat, his illness having put an end to such endeavours long ago, but he had still been around for a long time, learned quite a few tricks during his life and could more than compensate his technical limitations by putting sufficient spirit energy into it. As a result, what happened when the overstretched arrancar got kicked in the stomach can best be described by comparing the results with the particularly explosive uncorking of a champagne bottle.

The hollow was sent flying straight through the building at the end of the street. And the one behind. And the next. And the next.

By the time Ukitake reached the end of the debris trail, the false Kaien was just starting to get up. It appeared that his armored skin had blunted the worst of the attack, but he was clearly on his last legs. Blood dribbled from his mouth and nose.

"D-damnit, sir. L-let me *cough* release... my s-sword. You'll see, then."

The captain regarded the creature, forcing himself to remain dispassionate. At length, he answered with a minuscule nod.

What he was going to allow was stupid, profoundly so. Perhaps he would pay for his stupidity. Perhaps others would pay, which would be even worse.

Allowing the arrancar to release would heal most or all the damage he had already suffered. It would also give him new powers of unknown nature and magnitude.

But it would also alter the arrancar himself, bringing him closer in appeareance to the hollow he had been. It would reveal the true monster within and give Jushiro a chance to cut down his enemy without being haunted by the memory of Kaien...

"S-surge through the seas and rage through the skies, Nejibana!"

And before the dumbfounded eyes of the captain of the Thirteenth Division, the arrancar's soul-cutter started twirling, glowing and transforming into the familiar form of a trident, a weapon Ukitake had known nearly as well as the man who had wielded it.

"Convinced now, sir!?"

III- Ghost

The form he could doubt.

But the feel of joyous recognition in his own heart of hearts, as Sogyo no Kotowari reacted to the presence of a kindred spirit?

No. There was no mistake.

It was Nejibana.

But... that didn't mean that the one wielding it was its true owner.

"As a matter of fact, far from it."

"What!?" shouted the figure in white, raising the trident in a defensive stance.

"You look like Kaien Shiba, you talk like him, you move like him," answered the shinigami captain without moving from the spot. "But those things can be copied or imitated. They mean very little, moreso because the last time I saw the person you resemble he was being eaten from the inside by a hollow that had possessed him. So tell me what should I think. Is this a miracle? Or just another of Aizen's blasphemies? Are you Kaien Shiba or just a maggot wearing his flesh?"

Jushiro took one step forward. The other flinched, but stood his ground.

"If you think that I am a fake, explain Nejibana! You cannot copy a soul cutter, anymore than you can a soul!"

"Maybe. Or maybe you can actually copy souls and Aizen has found the way to do so. Or maybe I am just being decieved into thinking that I am seeing something that isn't really there. The traitor has a deft hand with illusions, after all."

"Screw that! Aizen is not even in this dimension, so he cannot touch you with his bullshit illusions. If you are going to double-guess the validity of absolutely everything you perceive, you are never getting anything done! You are gonna need to start trusting your own senses at some point and right now is a good time to do so, because paranoia is not conductive to getting shit done," shouted the arrancar, looking genuinely upset in a very Kaien-like fashion. "Besides, if there are illusions capable of fooling you in play tonight, then you have pretty much already lost."

"Excellent point. So you think that I should just cut in half what I think to be an imitator created by Aizen to exploit what he guesses to be a hole in my armor and carry on with my other duties?" asked Ukitake, while making a show of tapping his chin with a finger, as if absent-mindedly thinking about it.

"Well, no. When you put it like that, I don't. So... seeing that Nejibana didn't do the trick, how do I convince you that I am who I claim to be?"

"An explanation would be a good start."

"As you wish. The hollow that nearly killed me, Metastacia, was an artificial thing created by Aizen."

"I had guessed as much."

"Yeah?"

"Its powers were odd. Soul-cutter destruction and possession of spiritual entities are powers too specific to have developed in a natural way. But continue, please."

"Well, it was Aizen's creature, so its body was designed to automatically travel to Hueco Mundo after being killed, so that its corpse may be collected for study. Since he was inside my body when he died, I was also transported to Hueco Mundo. Also, rather than collected by the traitor, the corpse was found and eaten by a particularly powerful gillian, who was later recruited and turned into an arrancar by the traitor."

"And how do we go from there to the situation we are now in, if I may ask? Being killed, then eaten by hollows is not the kind of thing you usually come back from."

"That is not dead which can eternal lie. And with strange aeons even death may die."

"I beg your pardon?"

"Just something that came to mind. Seemed relevant. In any case, my soul still existed within the arrancar, dormant but undigested, and my soul-cutter spirit was tied to it. Aizen recruited that gillian because he was able to detect a strong shinigami power inside the thing."

"So?"

"The traitor has long been interested in the combination of hollow and shinigami spirits, so the arrancar was a test subject in many experiments. The latest involved giving him an asauchi and telling him to try to manifest Nejibana."

"You realize that this story so far does not actually increase my confidence in you being who you claim?"

"And the arrancar tried. He used a lot of spirit energy to try to reawaken Nejibana. It did not work, but it did wake me up and the last attempt made me strong enough to replace the arrancar as the dominant soul in this body. The moment that happened, Nejibana returned to life and the arrancar's soul-cutter exploded. The traitor just assumed that rather than finding a balance between shinigami and hollow, the last push turned the arrancar into an almost shinigami too far removed from his hollow roots to sustain a resurrección. He called it a failure, but failed to realize the change at the helm in here and moved on to other lines of research. That happened two and a half weeks ago."

"Now, that I don't believe. At all."

"Damnit, sir, you aren't making this easy!"

"I certainly hope so. Your exceedingly incredible tale only holds water if I am willing to believe that Aizen is a moron of the highest order, which I am not. He is a sick, evil soul, but he is also brilliant. So I have to decide whether you are a fake or a traitor. Neither of these is particularly palatable and I don't feel inclined to make things easy either way."

"So... why haven't you cut me in half yet?"

"A really good question. Mostly, because there is a chance, even if it is a vanishingly small one, that you are being truthful and there is a somewhat higher chance that Kaien survived, got captured by Aizen and used in experiments that twisted him into becoming whatever you are."

"This is still shit, sir, but thanks for the thought. So... what do we do now?"

"If you still want to convince me you are who you claim to be, you will drop your weapon now, submit to binding without resistance, and start telling me anything you know about the arrancar forces that might be useful. Otherwise, we go back to fighting and, seeing how it has gone so far, you lose."

"Since you put it like this... I surrender, I guess. Just one condition."

"No conditions. This is an ultimatum, not a negotiation."

"A plea, then. Guard me yourself, hand me to the Second, send me gift-wrapped to the Captain Commander... Whatever. I don't mind what you do, just don't leave me with the freaks here. Vivisection is not in my to-do list for today."

After a humorless snort, Jushiro nodded.

Kaien breathed deeply, closed his eyes and then threw Nejibana away. The soul cutter landed noisily, its trident form already fading away into the plain blade of its sealed state.

The shinigami captain had started uttering the incantation for Rikujokoro before the weapon hit the ground and, as six rods of light imprisoned the maybe Kaien, Ukitake felt the knot of tension around his heart relax somewhat. He allowed himself a sigh of relief, before continuing with the incantations of stronger, more comprehensive binding spells.