Bythos (深圏 (バイトス), Baitosu; Greek for "Depth; Profundity", Japanese for "Deep Sphere") is the dimension in which Nero and Drakosmima usually reside. It lies in between the Hueco Mundo and the Human World.
A dimension of infinite ocean, the sky is littered with clouds and a raging sun on a good day, but can turn dark, sinister, and stormy on a bad day. The landscape of Bythos is a seemingly never-ending light-blue ocean. In Bythos, the sun rises and sets opposite the human world, and at night, the stars never remain in the same position. The vast sea seems to lack any masses of land. Like in Soul Society, the atmosphere is filled with a high concentration of Reishi, thus allowing smaller Hollows to gain nourishment despite the lack of Human souls.
What appears as ocean is in fact a mass of super-dense Reishi. The Reishi is so thick and condensed, it appears as a liquid. At deeper depths, the pressure increases, and the light of the oppressive sun decreases. The “bottom” of Bythos is pitch black, the light of the surface completely swallowed by the Reishi sea above.
Similar to the case of Human World oceans, active life in Bythos is varied and ubiquitous. Nero generally dwell all over the ocean, constantly fighting for survival with their surroundings and each other.
Bythos is located between the Human World and Hueco Mundo, though it is separated from the bordering dimension known as the Dangai. It can be accessed by traversing the Antirevma, a void between the worlds which can be crossed by Nero, allowing them to travel to any other dimension at will.
Similar to Hueco Mundo, the entirety of Bythos isn’t united under a single government, but rather by a sort of situational strength hierarchy.
The darkest zone is at the bottom of Bythos is known as Nychta Stroma (無限夜海 (ニックタ・ストロマ), Nikkuta Sutoroma; Greek for "Night Layer," Japanese for "Infinite Night Sea") and only the strongest Nero survive here.
Tos Pyrgos (竜宮城 (トウス・ピールゴス), Tousu Pīrugosu; Greek for “The Towers,” Japanese for “Dragon Palace Castle”) is the fortress-state located beneath the deepest depths of Bythos, and the headquarters and base of operations of Typhon and the Diamanti. Tos Pyrgos resides in a realm of light beneath the darkest depths of Bythos. While it appears as a monstrous castle or palace, it is more of a city-state or polity that a structure. Tos Pyrgos is home to the most humanoid of the Nero, the Drakosmima.