Heartless Isolation. It's a technique that becomes available to Batsu'unsai whenever she releases her Bankai. It creates a boundary around Batsu'unsai that separates whatever is inside from outside interference. It sounds like a very simple ability but it's far from it because of the conditions to activate it and the way it interferes with the world itself.
To deploy Heartless Isolation, there needs to be a mutual agreement between parties that the opposing party must be killed. If there's anyone who doesn't share this sentiment, this barrier won't activate so it's useless against fleeing enemies.
But while it has a very specific activation condition, it's very effective in what it does. It doesn't block interruptions, instead, this barrier 'avoids' them by placing those inside within another dimension. Any attacks aimed at the barrier from the outside will just phase through it.
Meanwhile, those who are trapped inside won't be able to go out as long as the 'agreement' is not fulfilled.
This is the reason why Saizou thought that Batsu'unsai was shooting herself in the foot since there was no way for her to leave alive by trapping an immortal like him.
"Truly... I want to know what you're planning to finally kill me off." Saizou muttered as his body slowly changed with his hunched back going straight and his flesh gaining muscles. In addition to that, the purple mist was being ejected from his skin pores and slowly mixing with the thick fog around him and filling the limited space that they were in, "It may not be easy to find you, but as long as I'm alive, it won't be long before you drop dead."
Saizou's Zanpakuto on the other hand, also started to change as it wriggled like a living being before slowly transforming into a giant centipede that wrapped itself around him. And like Saizou, it's also emitting purple fumes in addition to the purple liquid leaking from its mandibles.
Just from his transformation, it's fairly obvious what his abilities are.
Before he could do anything drastic, Saizou felt something from his chest and when he looked down, there was a blade sticking out of him despite not feeling any presence behind him.
Giant insect claws instantly extended from Saizou's right arm as he swung it behind him. Unfortunately for him, there's no one behind him and even the spear that wounded him was gone. If not for the black blood spilling out of his chest, he would have thought that it was just a bad dream.
Even then, Saizou was not impatient since he knows that this domain that she created will eventually become his.
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On the peak of a certain mountain, Kuruyashiki was also watching what was happening in the distance. With various flashes of lights and signs of Bankai and Resurrección releases adding more to the chaos and heaviness of the atmosphere, he couldn't help but mutter, "Should have I accepted the fight with Unohana Retsu?"
He's now regretting the decision he made to stay away from them, though only until now when he suddenly felt a threat and used Flash Step to get away.
In the next instant, a flash of light appeared in the corner of Kuruyashiki's eyes before the peak he was previously standing on was suddenly obliterated by an explosion.
Landing on one of the trees not far down the mountain, Kuruyashiki looked up and saw a floating young girl with four pairs of differing wings behind her. A large grin appeared on Kurayashiki's face, "Finally! I was getting worried that you wouldn't show up! And why the sneak attack? That's not very honorable, is it?"
Claire, while holding her bow, replied, "I thought it was just appropriate with your Zanpakuto ready to get a bite out of me."
Kuruyashiki's grin grew larger and a mouth full of razor-sharp teeth silently emerged behind Claire, about to swallow her whole.
A staff of light instantly appeared in between the mouth and stopped it from closing. However, it didn't end there as a second mouth appeared on Claire's side.
This time, Claire didn't attempt to block and just flew forward. But in doing so, another, much larger gaping mouth was about to chop her down from the front. Claire could attempt to block it but feeling the staff she conjured as sturdy as she could be broken, she knew she wouldn't last long against it.
Nevertheless, instead of blocking, Claire had a much more effective method. She pulled back her bow and a black and white arrow manifested in it as she aimed at Kuruyashiki.
Kuruyashiki was surprised and thought she wanted mutual destruction. But when Claire suddenly turned her head towards the incoming mouth and her silvery grey eyes shined, it was then that Kuruyashiki recalled something and used Flash Step to get away.
Claire let go of the string while the mouth froze in place.
The area that Kuruyashiki was formerly standing was instantly eradicated by a huge explosion and the mouth that Claire stared at slowly returned to nothing.
Claire didn't let up and she deployed multiple Magic Circles and javelins of lights that started raining down on the general vicinity. Claire couldn't catch up to Kuruyashiki with his expert use of Hohou. She could technically match his speed with her own version of Flash Step, but she lacks the maneuverability, flexibility, and experience so an attempt would only expose her to more dangerous attacks.
On the other hand, Kuruyashiki continued manifesting mouths around Claire in retaliation but they only suffered the same fate as the first one that was frozen.
'What the hell is that?' Kuruyashiki asked himself while dodging the continuous onslaught. He tried just tanking one of the javelins and it easily pierced through his Reiatsu-strengthened skin though that's not the real problem but the fact that the wound it caused was healing ten times slower.
Kuruyashiki kept his distance and observed Claire and her actions and what he found was, 'The bigger the attack, the harder it is for her to nullify Gagaku Kairou (Corridor of Hungering Music).'
'That bow is dangerous. It could probably kill me if it hits me enough times, but I won't be going down without a fight!' After a few more minutes of observation, Kuruyashiki went back onto the offensive with a crazed grin on his face, forgoing most of his defenses, 'Let's go out with a bang! Don't disappoint me, young lady!'