Stormcaller hasn't gotten there yet, but here is the quote.
https://www.reddit.com/r/bleach/comments/ac9iqx/cant_fear_your_own_world_iii_pages_107135/
"Ancient times.
When the spiritual world had still not conpletely taken its current shape; when the Shinigami had no weapons to call their own; Menos Grande prospered within Hueco Mundo. During the dawning era of Hollows, their evolution was in a chaotic state. They evolved to the Adjuchas level. Unable to attain a humanoid form and without strong will, they continued to exist over a long period of time.
One of these spirits of old, which was particularly powerful, fought against Baraggan Louisenbairn and a few others who had reached a new form of evolution; a Vasto Lorde level Arrancar. It did not belong to any group and fought against Baraggan's faction's hegemony amongst the sea of sand in Hueco Mundo.
A creature possessing intelligence that continued to evolve, possessing a torrent of reiryoku. It kept devouring everything in its way, until one day it left Hueco Mundo and spread its influence towards the World of the Living and the reishi-enriched Soul Society; where the Shinigami were impotent. It continued to take in everything inside its own body, breaking down and destroying everything in its path like a hurricane. A grotesque figure with unlimited evolution. This resembled the ability 'Glotoneria' possessed by the Gilian which later came to be known as the Noveno Espada, but that is a different story.
Unlike the aforementioned ability, it could not retain the abilities of those it had devoured but gained a proliferative ability. Instead, the reiatsu of the ones it ate were inexhaustively accumulated within its body. And thus its reiatsu, compressed within the body of an Adjuchas grew until it came to be recognized by Baraggan, who governed 'senescence', as a 'walking disaster with infinite evolution'. The two of them reached an agreement to never interfere with one another.
Then, one day, something happened. At the end of its battle with the Shinigami, including young Yamamoto Shigekuni, it ascended to the sky by devouring the Rei-o, but was defeated by the monk and sealed by Nimaiya Ouetsu.
It was given the name 'Ikomikidomoe'."
Ignore this bit here
"it ascended to the sky by devouring the Rei-o" it was corrected in the comments saying that he was trying to eat the reio in order to ascend into the heavens I believe.
Even if it's argued it's too vague, Yhwach stated that all things connected to Soul Society would be destroyed. The palace itself is in Soul Society but in a different dimension.
