Overview
The Shackles are a mysterious ability possessed by the Chief of the MBCC. They are an absolute restraint to Sinners, enabling the user to sense the location and abilities of Shackled Sinners and providing a wide degree of control over them. The Shackles are also notable as the only known way to reduce an individual’s M-Value. Initially, this effect only work on Sinners, having no effect on regular Maniacs. Possession of Shackles also allows their wielder to consume Corpuses in order to become stronger. By the time the Chief had absorbed three Corpuses, their Shackles had grown in power enough that they were able to mitigate the effects of Mania on regular humans as well as Sinners. In addition to this growth, the Shackles' strength is correlated with the level of Mania contamination in the user's surroundings. In the most extreme circumstances, such as inside a Black Ring, their power is nearly unsurpassed. In an area with controlled M-Values, they are effectively useless.
Use of the Shackles is characterized by a red light that appears in patterns over the skin of both the Chief and the shackled Sinner, as well as in the environment surrounding them, which causes scorching pain to both the Chief and the Sinner, although the Chief seems to become more able to resist this pain over time. When Shackling a Sinner, the Chief hears a voice from the Shackles, a "cry from the dark." Shacking a Sinner requires direct skin-to-skin contact. How direct contact is defined is not entirely clear; they had no effect when the Chief touched Iron's prosthetic arm, but were able to work through Labyrinth's presumably artificial body. Once a Sinner is shackled, the only removal condition is the death of either the user or the shackled Sinner.
The power of the Shackles is intrinsically linked to the mind and emotions of their wielder, meaning that a significant shift in identity can have major repercussions on their usage. For this reason, the Chief has no power over Sinners who were Shackled by them at a time before they lost their memories, but is unable to reapply the Shackles to them in the present because the old Shackles are still in place on them. Currently, the only Sinner known to be constrained with the old Shackles is Shalom. The Chief is unable to even sense these Shackles' existence except when attempting to reapply their current Shackles to her. Likewise, when the Chief forgot their identity while their consciousness was wandering in the Nightmare Space, they were unable to use the Shackles on Dreya until they remembered who they were.
The Shackles may also have some independent will of their own. When the Chief met Parma for the first time during BR-004, the Shackles activated automatically and held the Chief in place. Parma stated that the Shackles were protecting the Chief and warning them not to get too close or be a hindrance to him. This phenomenon did not repeat when the Chief and Parma faced each other again inside BR-001.
History
Although the true origin and nature of the Shackles is mysterious, their power originated with the Underground. The first known wielder of the Shackles is the Underground Shepherd. They are said to have been born from the Lone Island in the DisSea, from where they were stolen by Paradeisos during the Eclipse Operation in N.F. 103. Shortly thereafter, Paradeisos enacted the Shepherd Plan to create Shackle holders of their own. They are described as the "ultimate container" for Mania, and a pathway that, by feeding on the Black Ring, can ultimately lead to the Source. Nightingale refers to the Shackles as a "forbidden power" that laid "dormant in the depths" of the prison until awakened during the MBCC prison break.
In addition to the Shepherds' Shackles, Paradeisos also used the power recovered from the Lone Island to develop a derivative ability, the Mark.
Powers
Combat Ability Enhancement
The Shackles provide their wielder with a variety of combat powers, seen most clearly in the various Potestas that the Chief wields. The Chief also frequently uses them to fire an energy-based projectile weapon, often described as a blast of red light.
Weak Sense Tracking
The Shackles allow the Chief to understand the abilities of any Shackled sinners, as well as sensing their physical presence and direction to them, even over great distances. This ability is two-sided, allowing Sinners to also detect the presence of the Chief, although the sense seems to be fainter when used in that direction.
The Forbidden Force
The user can absolutely forbid specific behaviors with clearly worded orders. The Chief is also able to control the world of Shackled Sinner's spiritual realm (6-4), which may be another function of the Forbidden Force. Although this ability is said to be an "absolute restraint to Sinners", it is often seen that Sinners are able to stop or resist the Shackles when the Chief is under their mental sway, such as when they are hypnotized by Chameleon or in the illusory world of Hamel. The Forbidden Force also loses effectiveness as a Sinner comes closer to transforming into a Corruptor, as in the case of Ninety-Nine.
Mania Mitigation
The Shackles are the only known way to reduce one's M-Value. Sinners fighting under the effects of the Shackles have their M-Values rise as usual when using their abilities, but they quickly return to their previous levels once combat is done. Although this ability initially only works on Sinners, as the Chief's power grows, they eventually become able to treat Mania symptoms in regular humans as well.
Mental Feedback
The Chief receives mental feedback from those they Shackle, which most often manifests in dreams of the Sinner's past or secondhand mental corruption when the Sinner experiences extreme effects of Mania. This corruption feedback frequently causes nausea, vertigo, and even unconsciousness. When a Sinner is experiencing extreme emotions, the Shackles often burn with painful heat. Positive emotions can also make them hot, but in a warm and gentle way, as when Zoya remembers Leggett during BR-004. This connection works both ways, and it is possible for contamination from one Sinner to pass through the Shackles to the Chief, and then from the Chief to further affect other Sinners.
Although the positive powers of the Shackles have no effect on non-Sinner Maniacs, attempting to Shackle a Maniac in the process of turning into a Corruptor still caused the Chief to become dizzy and vomit due to mental feedback.