Universe Onyx

Universe Onyx (宇宙オニキス, Uchū Onikisu) is an alternate timeline that strays from, the main timeline of ' and '. According to and, because the color designations are closely related, these two timelines are supposedly very similar to one another.

Overview
Within the, there are an infinite number of timelines which manifest as alternate universes. As theorized by, and later confirmed by research done on the matter by the , every particular timeline emits a specific wavelength that corresponds to an individual color band which serves as its marker. Just as there are an infinite number of colors, so too are there an infinite degree of universes, and thus every possible timeline has its own unique color assigned to it. Based on how distant a color is from another color on the spectrum, that determines how different the timelines happen to be. Universes exist which could have absolutely no similarities with another, while to the opposite extreme, some timelines can be extremely similar to one another. Universes which share a color designation that happen to be different shades of the same color, such as Black and Onyx, are those that differ only in a couple of minute aspects. However, even those small differences can result in the inhabitants of one universe having lived entirely different lives than their counterparts in the other universe.

Gotei 13
In Universe Onyx, there also exists a rendition of the. Unlike in the Black timeline, the Gotei 13 is annihilated by and the  on the first invasion. Only few of the original Captains at that time survived and managed to escape as the Soul Society was seized by the Quincy King. Among them was, who stepped up and took command of the remnants of the Shinigami military, eventually leading to centuries of continued war that finally led to victory for the Shinigami. Following the war's conclusion, and the peace that followed, Captain-Commander Seireitou worked to restore the Gotei 13 to its former glory by restructuring the Shinigami academy, eventually managing to recruit new soldiers that would be trained by the older veterans. Though it took time, the Shinigami organization had blossomed once more as the dominant authority within Soul Society.

Only five veteran Captains, besides Seireitou, survived the massacre centuries ago and subsequently took part in the recapture of the Seireitei. Among them, took charge of the Second Division and became the Commander of the Onmitsukidō, though he often delegated his Captain duties to his Lieutenant while he himself took part in dangerous solitary missions. Though children at the time of the war, Kyōraku and Ukitake would eventually become Captains of the Eighth and Thirteenth Divisions respectively. Kisuke Urahara still took up the post of Twelfth Division Captain, maintaining that post even as of today.

Behind the Scenes
This is just an idea in the works, the author is only carrying out this concept on a trial basis for now, so please be patient until the idea is either fully fleshed out or eventually scraped.