Through the 8th door, Shinji entered alone for a very good reason. It's because his ability is one of the weakest blades with comrades around, but one of the greatest weapons when uncountable foes surround him.
The moment he found himself in the foreign world, he couldn't help but smirk at himself, "My~ Isn't this convenient?"
In front of a legion of Hollows with holes outside of their bodies, also known as Beasts of Hell, "A stage set just for me, huh?"
Placing his Zanpakuto in front of him, his Reiatsu rose up to the highest level, making the surrounding beasts more aggravated, "...Bankai. Sakashima Yokoshima Happoufusagari (Reversed Evil Eight Treasure Blockade)."
The moment he finished muttering the release incantation for his Bankai, his Zanpakuto instantly transformed into a staff with its front end turning into a large ring while a flower construct bloomed under Shinji's feet and its petals covered his body as if to protect him from any outside harm.
In an instant, the atmosphere around changed and the hate-filled air became even denser. But unlike earlier, none of it is now directed at himself as the beasts turned against each other and started devouring their comrades, turning the whole place into a bloody feeding frenzy that no normal person would be able to stomach.
Sakashima Yokoshima Happoufusagari is a wide-area Bankai that targets opponents and allies alike without discrimination and without exception. It inverts the perception of friend-and-foe across anyone within the vicinity of his Bankai, with Shinji himself being the sole person unaffected. As a result, all allies become enemies while all enemies become allies for the duration of the Bankai. As such, it's one of the best weapons against the current situation and the reason why he's alone.
But while he's being protected from the rain of blood and flesh by his petals, a panicked voice of Akon from his communicator rang out, [Door 6 and 7! Retreat! I repeat! Door 6 and 7! Retreat! As fast as possible!]
At the same time, Shinji watched the door he went through close, "...What's going on?"
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While the groups who entered the 9th and 8th doors met thousands of Beasts of Hell and it could even increase from that since more are gathering around, the 1st through 7th doors are vastly different with only hundreds of Beasts of Hell in them, and sometimes, none at all. But in exchange, there were [Chained] individuals waiting for them.
Through the 1st door that Byakuya and Renji had entered, the one who's waiting for them was the sole Yammy Llargo. Although they were already expecting to meet an Arrancar, his presence still couldn't help but elicit a frown from both of the most vital members of the 6th Division, "Be careful. If he retained the amount of Reiryoku he used to have, he's already dangerous, but if he gained any other abilities apart from his brute strength, it's going to be a problem." Byakuya warned Renji.
Renji took his warning to his heart but also said, "It's alright, Captain. It's not just you who's improved, but also me."
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Ichigo's group entered through the 2nd door and immediately came across hundreds of beasts. However, what caught their attention is the detestable man wearing a disgusting grin, "Welcome, Ichigo Kurosaki and his merry friends~! To my kingdom of special beings!"
"Just our luck..." Uryu muttered to himself while staring at the thin man with shoulder-length pink hair, "...Or maybe not."
Szayelaporro grin grew larger, "Correct, Ishida Uryu! I knew that you guys were going to come, so I made it so that any door you take will lead you directly to me!"
Uryu fixed his glasses while returning his provocation, "That's awfully confident of you. Do you really think you can take the three of us? We're not the same ones you met years ago in Hueco Mundo."
Nevertheless, Szayelaporro was unperturbed and immediately noticed Uryu's actions and intentions, "That's just like you, Quincy, looking for what looks like traps and setting up your own. You really haven't changed as much as you claimed."
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From there, the Quincies met Nnoitra Gilga and Zommari Rureaux past the 3rd door, SoiFon, Marechiyo Omaeda and Yumichika Ayasegawa met Baraggan Louisenbairn past the 4th door and Zaraki Kenpanchi met Kaname Tousen and the former Kenpachi, Kiganjou, past the 5th door.
However, Akon's panicked voice who was watching from the monitoring room was caused by the people waiting at the 6th and 7th door.
As the Hollows entered the 6th door, they were instantly welcomed by a massive Reiatsu pressing down on them, and it was the same situation with Lisa Yadōmaru, Yuyu Yayahara, and Tatsufusa Enjouji when they entered the 7th door.
Since the people in the monitoring room studied to prepare for eventually meeting the people of the past. Akon immediately issued a retreat order after identifying the people waiting for them. From the 6th door, they were the former Kenpachi, Kuruyashiki, and one of the founders of Gotei 13, Saizou Sakahone.
The 7th door, on the other hand, also had founding members of the Gotei 13, Nobutsuna Shigyou, and Chigiri Shijima.
Akon immediately judged that the two groups would just be throwing away their lives if they fight them now. However, his retreat notice was a tad too late as all the doors closed on their own.
Realizing this, Akon slammed the table in front of him, "Fuck! They were already expecting this! To all units available! Gather around the gates and do everything in your power to reopen them! Where's the Captain!?"
One of his subordinates immediately answered, "He's already on his way to the Human World."
Akon took a deep breath to calm his nerves, "I see... At least make sure we don't lose our connections with them! Increase the power output to supplement the Negacion threads' integrity! Someone contact the Arrancar, Roka Paramia, and ask for her assistance in case we can't keep the connection open!"
The entire 12th Division immediately jumped into action the moment things went awry.
Nevertheless, they weren't the only ones watching it in real-time as Mayuri also provided each Division monitoring equipment to watch the whole operation, though it's mostly for the Captains who can't leave the Soul Society to secure it.
Seeing the situation, Shunsui immediately took out his phone which was modified some time ago to contact someone. As he placed the phone on his right ear, its special function of silencing everything around him immediately activated. He heard his phone ringing a few times before it was answered, "Just those two doors. Please save them."
[I'm on my way. But is it possible for you to delay Kurotsuchi-san for a bit?]
"No. He'll immediately be suspicious of me if I do that."
[Oh well, that's fine. It'll be a bit easier for me if I don't meet him now.]