This was an idea I came with around 2011 after seeing Bleach: Hell Verse in Japanese (I was too impatience to wait for the subbed version). So nothing that has been revealed about the true origin and identity of Zangetsu will apply in this story. 0000000000000000000000000000000000000000000000000000000000000000000000000000000000000
After having gone berserker in his new fully hollowifed form, Ichigo was stopped this time by Renji doing the very same thing that Ulquiorra had before him. He had sliced off one of his horn then used a the special kido that Rukia had entrusted to him to use just in case something like this had happened. Once that had been done that left him, Rukia, and Uryu to alone with a very pissed off Kokuto. After all he already started losing after finally having his chance at freedom most because he had literally gone surfing on monstrous powerful Cero that was able to break Hell Gate. Now that after being denied his freedom because someone had stopped him after just having one single chain left. Well Kokuto took out his angry on the three of them until they finally started be overcome by the effects of Hell. Renji and Uryu were hung from tree as pair of drying out mummies having not completely become a member of Hell yet.
However the same was not true of Rukia as Kokuto may have become aware of Ichigo and his fully hollowified form's massive Cero during his final battle with Ulquiorra. However rather then simply attacking him the moment he got the chance to finally be free of Hell. Kokuto spent his time learning up on Ichigo himself and those around. Having played the puppet master as the one that feed the images of his power and false hope that simply destroying Gate of Hell across the various levels until Shuren and his group took the bait.
During this time he learned those that were closest to Ichigo. Rukia was the second person one that list along with Orihime with only his family being ahead of them. Rukia because she was always there to back him up no matter what went down around him while Orihime was the reason he transformed into his full Hollow form in the first place. He did so because he was trying to save her from Huenco Mundo and Ulquiorra was standing the way.
So while he went easy enough on Renji and Uryu (who he was rather surprised that he was lasting so long despite Hell's environment being toxic to living humans) Rukia was not on the receiving end of such mercy. Using the same move he used on Ichigo sending his sword straight into her chest that caused a massive explosion to happen in the distance. Although rather then the explosion happening in the distance it happened with her being at ground zero.
He waited for awhile then Rukia now part of Hell just like him had been pulled at that molten vat dressed white robe (the same one went she was to executed for giving Ichigo her powers) with chain connected to her chest. The chain being proof that she was now a member of Hell. Thus Kukoto while waiting for Ichigo to either come back or for him to simply tired of waiting. He decide to just repeatedly kill Rukia over and over again.
Kukoto was unaware that Ichigo was not the only one with monster residing within them. There was one other with a monster sleeping within them that was being aroused from it deep sleep by his actions.
He was just about to kill petite black hair girl before him she teleported away leaving him to strike only air.
That when Kukoto noticed that despite being in area of Hell that was suppose to be rather hot it was suddenly getting cold….very cold. He then moved out the way as Rukia suddenly appeared behind and lunged for him missing him by only inches. However she then appeared behind him again slammed his head into the ground before kicking him in the butt launching him dozen yards away.
Kukoto managed to right himself in mid air as he noticed that Rukia had become almost white in skin complex and she was swaying from side to side before once again she vanished. Appearing once again from right in front of his path she swung what appeared to be a scythe made completely out of ice.
Turning it around with amazing speed for something that large she swung at him again though this time there was energy wave that was release from the scythe's blade that was looked like black ice. The white hair sinner was just about to block the move with his remaining chain and his own power yet something within him told him that would not be bad idea. Good thing he listen to that feeling as the blast frozen solid Shuren who had revived and was looking to get his revenge on Kukoto for killing him before he fully reviving earlier.
Wondering just what the hell had happened to Rukia as she was not suppose to have a transformation like this nor be this powerful either. Of course what no one realized even Rukia herself was when she was undergoing that rather unique form of Hollowification after being possessed by her two old friends. Ichigo may have managed to halt the process by giving her his own powers thus killing the Hollow behind everything. This did nothing to kill the Hollow that had been born and taken root within Rukia in similar manner that gave birth to Hollow Ichigo.
Thus Hollow or Dark Rukia remain hidden within the depth of her being to the point where even Rukia's own zanukpato was unaware of her existence. Yet she also served as buffer to keep her dark side locked away as Sode no Shirayuki was much stronger presence within Rukia. That ended when Kukoto had made Rukia a member of Hell effectively stripping her of her shinigami powers or at the very least extremely weakening them. Thus Dark Rukia was able to bubble up from the depths of Rukia's being having been aroused from her slumber through the constant deaths Rukia was being put through.
Rukia's basic Hollow form was just like her original Dark Rukia one, with her hair turning purple, skin becoming white, and eyes yellow and demonic in appearance. Her clothes had morphed to mixture of what her zanukpato and her Dark Rukia form. Her weapon was scythe yet in turn with her ice based abilities it was made completely of ice.
As she stood there seemingly well out of reach to attack Kukoto her weapon even her teleporting ability or energy attack. A smirk appeared on her face as then just suddenly appeared behind him her eyes completely black as she swung her scythe beheading the white hair sinner yet it did not stop there. Appearing to catch his head she hurled it at his now frozen body shattering it to pieces before squashing his head like pancake with her scythe's blade flipped on it side.