Hueco MundoHueco Mundo is the abode of hollows (sometimes called "hungry ghosts" by yokai), the spirits of people who have been overcome by negative emotions and become cannibalistic monsters with a hole somewhere on their malformed spirit body. It is a world with highly concentrated reishi (spirit energy) that the hollows feed on. The surface of Huecho Mundo is a sandy desert with the tops of crystalline "trees" poking up and some rocky outcrops and an eternally dark sky with only a moon (that displays the opposite phase of the world of the living), while the hollow place under the sand is a dark crystalline "forest", the abode of the more powerful hollows, like the menos grande, gillian class, or adjuchas. The surface contains the palace of the arrancar (powerful hollows that have been made similar to soul reapers by the Hogyoku, like the espadas, who essentially rule Hueco Mundo) called Los Noches, while the underground forest called the Forest of Menos contains Sulis's Grotto, the place where the soul of the deceased great fay Sulis lives, dripping life water (the bane of the dead especially hollows), along with the stranded soul reapers gathered by her attendant the soul reaper Ashido Kano.
This Hueco Mundo was pulled from the Hueco Mundo of the Bleach Universe at the fall of Primavaldia sometime after the events in the life of Ichigo Kurasaki had passed. Though none of the original arrancar were pulled into Primavaldia the soul reaper Ashido Kano was. When Sulis died and was pulled into the newly formed Hueco Mundo Ashido Kano went with her rather than stay with the other soul reapers.