With Shinryu's return, a report on the infomation he learned in Hueco Mundo has been delivered and dished out to all the Shinigami.
5 years ago Hueco Mundo was ruled by Ashlei Clixx, who was mostly absent, and Rose Mischevang (her sister). Ashlei was a Throne in Shadow Fall and one of it's leaders while Rose led the Shadow Espada (technically/kinda) in Shadow Fall. At the time, Nagato Tengan was still an Infante in SF (which are kinda like the equivalents of Seireitei's Noble Houses in terms of authority and power). Years prior the strongest Arrancar alive had been Nagato Tengan, Zefonse Kaizme, and Ashlei Clixx; by this point Zefonse was gone and Ashlei was mostly afk, leaving a pretty significant void in their wake. Eventually Ashlei went AWOL entirely and Nagato moved up to become a Shadow Fall throne and leader of Hueco Mundo. However he was never one to share power, and left Shadow Fall. This resulted in the newer power of Rose Mischevang becoming the new SF head honcho in Hueco Mundo. Nagato ended up creating his own faction, the Hollow Empire, and basically became a bordering/competing nation in HM to Shadow Fall. The empire grew and grew and grew....then stopped. Nagato disappeared like Aang the Avatar, his faction split and became leaderless, and Shadow Fall/Rose remained unimpeded in Hueco Mundo for a solid 2 years. Not much occurred there during this time. Random people came and went, and the Shadow Espada became barren with basically only Rose at it's helm and nobody else around.
But over a year ago, Nagato reemerged from the depths of Los Viejo ready to strike as Shadow Fall teetered on the edge, with the org constantly dealing with new and more enemies. Taking advantage of the turmoil, Nagato brought in Kimimaro, his right hand man, and called together a force built by Hollows and Arrancar alike from all across Hueco Mundo. This force spent a few months growing and assimilating others, then in one swoop knocked Shadow Fall out of Hueco Mundo. Kimimaro sacked Hueco Mundo's capital and took control of it, Mischa eliminated and pacified outlying bases, and Nagato took on Rose Mischevang in single combat; all three ended as victories for his faction. With Shadow Fall out of the picture, Nagato renamed his force the Espada and brought back the original 0-9 numbering system, reformed the Privaron Espada to be about inclusion of any and all Hollows/Arrancar who wished to join up, and put all of Hueco Mundo under a single banner. At this point he reached out to Yaksha, a Hollow with a small territory on Earth, and when his olive branch was denied with words of war, Nagato sent some of his Espada to go wipe Yaksha and his territory off the face of the Earth. Recently, the overall quantity of Hollows and Arrancar that roam through Hueco Mundo has diminished however, more powerful than they have been in recent memory, with even the presence of Vasto Lordes becoming not so rare. As well, the overall quantity has become less scattered and more focused in certain areas.