(last updated: 12/28/17)
Some rare individuals are capable of channeling specific types of energy directly from other planes of existence. For example, characters like Kyo and Saishu Kusanagi are capable of pulling heavily from the plane of sealing fire that empowers their techniques, which is why their specific chi use is of particular interest to NESTS.
Grimoires are engineered to accomplish roughly the same remarkable phenomenon. A grimoire is a portable power source containing a specific type of energy. For example, a grimoire of fire would allow its user to syphon a phenomenal amount of fire to fuel their combat powers without having direct access to the elemental plane itself. Other Grimoires are tools, able to create a very specific phenomenon by drawing from or using energy. Given the limitations of Ars Magus and the rarity of true mages, Grimoires are an incredible military power.
However, like all man-made devices that meddle in interplanar powers, grimoire use often comes with unintended consequences. Some of these side-effects can be beneficial, such as acquiring additional quirks or enhanced senses, but most of them are negative. Channeling planar energy through a grimoire exposes one's soul to damaging energy that will almost always eventually result in the user's ruin. These wounds build up slowly like scars, and do not vanish with time. Every overexertion is permanent.
Grimoires do not have a specific physical form and are incredibly difficult to pass between users as they meld with a wearer's soul. Techniques exist to extract or seal grimoires in physical vessels until someone comes to inherit them, but otherwise once attached they usually a permanent part of that user's life until they die.
Having a grimoire can attract unwanted attention, as it makes one's soul much more potent and appealing for nefarious purposes. Grimoires do not have to exist physically once they are attuned with an individual. Some wielders might have no visible evidence of possessing a Grimoire. Others have replaced missing limbs or even an entire body with one. Similar to Nox Nyctores, Grimoires seem to have some level of sentience that causes them to seek out and bond to compatible users. It is however possible to seal unattached grimoires to prevent them from bonding of their own free will.
Grimoires are remarkably rare and date back to a time when the NOL absorbed a secret sect of powerful magic users known as the Ten Sages. Since that time, grimoires have been passed down mostly through the Twelve Originator families that founded NOL as a reward for their loyalty to the organization. There are researchers trying to discover ways to create new ones but this is a very dangerous area of magical and scientific endeavor and there have been many casualties along the way.
It is possible to apply for OCs that possess Grimoires, although their power level is still within the scope of starting ratio, allowing very specific power manifestations. The OC must be able to link the grimoire back to either the NOL or one of the Twelve Originator families. As with all grimoires, the OC's grimoire must come with both positive and negative ramifications. In all cases, having one can attract a lot of undesired attention. The NOL itself will aggressively pursue any individual in possession of a grimoire that is not numbered among their ranks.