Black Vox is a highly secure Mage Prison where extremely dangerous criminals such as Drum Bee are imprisoned. Its force of jailers is akin to a military unit. This coupled with the prison's incredibe defences makes Black Vox an almost unconquerable fortress, such that a direct assault upon it would be foolhardy. Due to this fact, it became the target of the Dark Guild Five Bridge Familia, as they were intent on making it their Guild's headquarters, even allowing their Guild Master to be captured and detained there for the sole purpose of infiltrating it.
The main structure of the prison is a gargantuan cube, completely black and opaque. It hovers quite some distance off the ground and is tilted a bit about its axis such that it isn't completely parallel to the ground. Although completely solid, a square opening can be created on its surface to allow passage.
The main cube is surrounded by much smaller ones, almost miniscule in comparison, whose appearance is similar to the one they orbit. These serve as guard outposts and appear to be capable of locomotion, able to intercept any suspicious objects that approach the prison. Unlike the central cube, these aren't completely opaque, having a transparent glass-like band on the surface, allowing the people inside to observe the surroundings for any suspicious activity.
The interior of Black Vox is designed in a simplistic manner; just one huge empty space with a massive cube floating in the center. It serves as the Central Control Tower and appears to be stationary. It seems to be assembled from various segments, each component having a clear outline. It is completely bare except for a mobile keyboard which is used to control the cells in the prison. The keyboard does not appear to be attached to anything.
There are numerous smaller cubes floating around the Central Control Tower, each serving as a cell for a single convict. These cubes are mobile and seemingly float around the space without feeling the effects of gravity. Their movement can be controlled using the keyboard on the Central Control Tower. The interior of these small cells is completely bare in some cases, while others contain various tools and equipment for the inmate's recreation requirements. They can also be opened by removing the walls and the entire roof. The cubes also seem to have their own artificial gravity as the prisoners they house are able to stand upright while inside.
Charged with countless murders, Drum Bee was captured and contained in the prison. In order to free him, his subordinates kidnapped Warden Torch and were able to blackmail their way into the prison, where they made a deal with Head Jailer Grog to exchange Drum for Torch. But on being released, Drum revealed his true plan: to take over Black Vox itself and make it his own Guild. Gray, who had also been sucked into this plot as he witnessed Torch's abduction, attempted to thwart their plans but to no avail. To make matters worse, Drum released the rest of the prisoners and encouraged them to slaughter their jailers.
In this desperate situation, Gildarts revealed himself to be among the released inmates, having taken that disguise in order to complete his job of taking down the Five Bridge Familia. He easily defeated the Dark Guild and then left the prison along with the young boy, the entire Dark Guild now safely apprehended and in the jailers' custody.