Sabikui Bisco, Vol. 4 (light novel)

The Six Realms Penitentiary is a legendary prison so vast and so intricate that many have come to call it the prison city. Hot on the trail of the Akaboshi Mark I, Bisco and Milo find themselves in Kyushu, and there they receive the troubling news that a mountain of a man swept through the area, snatched up Mushroom Keepers, and took them to this place of no return. Worse, the giant bore mystical flowers that steal the life force of anything they touch, including mushrooms! Now Bisco and Milo will have to infiltrate Six Realms Penitentiary and pull off the prison break of a lifetime, but they won't be doing it alone. A young "prince" who also possesses the ability to manipulate these ravenous blossoms is serving a sentence as well. But their path to freedom is barred by something much tougher than prison walls...