The Duodecim (十二宗家 Jūni Sōke), also known as The Twelve Spears are a set of twelve families. Their primary function is to supply personnel and soldiers to the Novus Orbis Librarium, and are the most elite of the noble families.
The Duodecim originally stood as a set of families that were deeply ingrained in the founding of the Mage's Guild and had historically managed the Magister's City of Ishana. Although separate from the Ten Sages, it was not unheard of for a Duodecim member to become a Sage. During the Dark War, these twelve families firmly grasped and mastered the concept of Ars Magus, becoming fundamental contributors to the battle against the Black Beast both as researchers and soldiers - two of these contributors were Seven of the Ten Sages, and Ayame Yayoi, one of the first wielders of the Sealed Weapon Izayoi. These families swore their allegiance to the NOL. Each family head of the Duodecim meet once a month to make important decisions, with the Imperator Librarius of the NOL also attending. This event is called the "Conference of Leaders".  Generally speaking, the heads of the family are the only ones who know the identity of the Imperator.
Primarily, the function of the Duodecim is to provide exceptional individuals to the NOL as personnel. Despite the Duodecim calling themselves "families", there are some of them which do not lend themselves to the traditional meaning of the word, such as the Kisaragi, who find and recruit talented children into their ranks. Generally speaking, however, different houses have different rules and traditions. The Yayoi, for example, place a massive importance on bloodline and the purity of it. Nearly all Duodecim children are sent to the Military Academy at a young age, and are given certain privileges and obligations. Privileges include precedence in enrolling into special classes and in enrolling into the student council. Upon graduating, all Duodecim members are guaranteed at least the rank of First Lieutenant; other privileges also exist. However, the obligations are strict. Duodecim heirs must act as role models, achieve perfect grades, and take central roles in school events and community volunteering; failure of the former two can lead to the individual's status being revoked.
The children that the Duodecim invest into the NOL are trained from a young age and nearly always excel in Ars Magus to the point where half of the children shown are capable of wielding Nox Nyctores. As well as Ars Magus, they are also trained in more traditional combat, excelling in martial arts. During the Ikaruga Civil War, many Duodecim members participated, and records showed that the vast majority of them survived the war.  Each family are made up of various elites and true nobles, and each family has various strengths and types of Ars Magus as a result. The Duodecim also have branch families which generally consist of assassins and those who handle the dirty work of the main family they belong to.
In the February of 2200, all of the Duodecim families had begun to rebel against the NOL under Izanami's rule, wanting the true heir of the NOL, Homura Amanohokosaka to take over instead. This rebellion was somewhat successful as Izanami abdicated and declared Homura her successor, but seemingly not because of the rebellion itself. It is unknown if some members of the Duodecim were absorbed into the Embryo, but those that were, were sheltered inside the Yayoi household with Homura. Hazama remarked that by this point, the Duodecim held very little power, and that it was no more than just a title.
- Mutsuki - led by Kagura Mutsuki. Head of the Duodecim as a whole, the Mutsuki are incredibly talented, although not much about them is known. They spearheaded the rebellion against Izanami to seat Homura Amanohokosaka on the NOL's throne. One of their branch families is the Kohaku. They currently watch over the 5th Hierarchical City of Ibukido.
- Kisaragi - led by Seven. They do not care for bloodline or status, and gather talented children across the world, giving those with obvious strengths an advanced education and combat training. Because of how they operate, the Kisaragi produce a great many individuals for the NOL, but it has led to a reputation of them being viewed as extremists and mercenaries by most other Duodecim families. They share a strong relationship with the Yayoi despite being their polar opposite. Jin Kisaragi is one member of this family.
- Yayoi - led by Tsubaki's father. The purity of their bloodline is all that matters to them, as it stabilizes their abilities and allows them to be more reliable. Because of their strict rules for marriage, the Yayoi's numbers decrease with each generation. Many of the other families believe they are on the verge of extinction, and the NOL has even requested they dissolve the rule, but they did not take this advice. Other Duodecim families view the Yayoi as borderline heretical and conservative to the point of extremity. Despite this, the Yayoi share a good relationship with the Kisaragi despite their ideologies being polar opposites. Tsubaki Yayoi is a member of this family, whose ancestor is Ayame Yayoi. The Yayoi live in the 4th Hierarchical City of Naobi, and they guard one of the few Sealed Weapon Izayoi left in the world.
- Hazuki - led by Houichirou Hazuki. Pure power is all that truly matters to this family, and not being strong can result in a member's status being revoked, or the member themselves killed. The members of the family that made an appearance in media are genetically related, but their relationships are complicated. One of their branch families is the Kirihito. Rin Hazuki and Mai Natsume are members of this family. They reside in the 11th Hierarchical City of Shinatsu.
BlazBlue: Central Fiction Library Entry
- Mutsuki Family
One of the families of the Duodecim. 'Mutsuki' means 'the first moon'.
The current head of the family is Kagura Mutsuki. They object to the Imperator's actions, and oppose the Librarium. 
- Kisaragi Family
One of the families of the Duodecim. ‘Kisaragi' means ‘the second moon.'
They attach no value to bloodline or social standing. One's abilities mean everything in this family. They gather talented children from around the world by various methods, and train them thoroughly in combat. 
- Yayoi Family
One of the families of the Duodecim. 'Yayoi' means ‘the third moon.'
Unlike the Kisaragi and other Duodecim, bloodline is the most important quality in this family. This strengthens their bonds, stabilizing their power. However, their stressing of pure bloodline also causes various problems, and their numbers are decreasing with every generation.
The current head of the family is Tsubaki's father. 
- Uzuki Family
One of the families of the Duodecim. ‘Uzuki' means ‘the fourth moon.' 
- Satsuki Family
One of the families of the Duodecim. ‘Satsuki' means ‘the fifth moon.' 
- Minazuki Family
One of the families of the Duodecim. ‘Minazuki' means ‘the sixth moon.' 
- Fuzuki Family
One of the families of the Duodecim. ‘Fuzuki' means ‘the seventh moon.' 
- Hazuki Family
One of the families of the Duodecim. ‘Hazuki' means ‘the eighth moon.' 
- Nagatsuki Family
One of the families of the Duodecim. ‘Nagatsuki' means ‘the ninth moon.' 
- Kannazuki Family
One of the families of the Duodecim. ‘Kannazuki' means ‘the tenth moon.' 
- Shimotsuki Family
One of the families of the Duodecim. ‘Shimotsuki' means ‘the eleventh moon.' 
- Gokuzuki Family
One of the families of the Duodecim. ‘Gokuzuki' means ‘the twelfth moon.' 
- Duodecim (The Twelve Spears)
Twelve families that quickly adopted ‘Ars Magus' during the Dark War, and contributed significantly. They are also known as ‘The Twelve Spears.'
They are deeply involved in the foundation of the Librarium, an elite group of contributors, if you will. They were formally named after the end of the Dark War by the Librarium, and each of their family names are tied to one of the twelve months. 
- All of the Duodecim's families are named after the traditional Japanese names of the lunar calendar months.
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