Hozuki Castle was a prison established by Kusagakure at the behest of various nations to hold criminal ninja. On the surface, it housed prisoners, but in reality, it existed to research the ultimate weapon from the era of the Six Paths Sage—the Box of Ultimate Bliss.
There were roughly three types of prisoners held there.
First were ninja abandoned by their villages or deemed to hold no intelligence value. They existed as experimental materials for researching the Box of Ultimate Bliss. These were the prisoners who could vanish without a trace at any time, and they made up the largest group.
Second were spies planted by various nations. Their primary purpose was to gather intelligence and monitor the progress of the Box of Ultimate Bliss research, so they could seize or destroy it once the research was complete. Kusagakure knew these people existed, but it was in no position to oppose them and could only tacitly allow their presence.
Third were actual criminals, mainly those captured by Kusagakure.
Hozuki Castle's current warden was Mui, a Kusagakure jonin.
Mui was skilled in Kusagakure Style taijutsu and a Fire Release sealing technique known as the Heavenly Prison Technique. This ninjutsu prevented its victims from gathering chakra. If they left Hozuki Castle or strayed too far from Mui, the Heavenly Prison Technique would activate and burn them to death. It was an outstanding method for suppressing prisoners, which was why Mui's family had guarded Hozuki Castle for generations.
After Mui's son, Muku, died in an experiment involving the Box of Ultimate Bliss, Mui came to his senses and became utterly disillusioned with Kusagakure. His only reason for researching the box was to resurrect Muku.
Unfortunately, while the Box of Ultimate Bliss appeared to be a weapon of war capable of granting wishes, it