The Limbo is the place where Reina has been imprisoned since the days of her exile. It is a place outside the control of both High and Low Spheres where are imprisoned extremely powerful and dangerous beings from both sides.
According to the High and Low Spheres if someone stays for too long in the Limbo, takes the risk of forgetting who he or she is and why is the reason they are there, therefor the person gets lost forever. Imprisoned inside one of the most powerful cells, Reina needed a sacrilege between an angel and a devil to happen so she could escape. Watching the world from simple video-windows, Reina targeted Raf and Sulfus as the ones who would free her from this place.
In the very end of Season 2, Blu appeared and was called by the two guards, who bowed before her, 'Princess Blu'. It is unknown why Blu is the princess of the Limbo and who are her 'people' that she keeps mentioning through the season.
AppearenceThe Limbo is a dark and gloomy place where there is nothing but long, labyrinth streets filled with green clouds and mist.
The mist and the clouds are the main cause of why many, who dare to enter inside the Limbo, get lost forever, becoming themselves prisoners of this place.