Eien Shinken (永遠神剣), Eternity Sword, lit. -Eternal Divine Sword) are sentient weapons that broke from a single Eternity Sword, Aeon, the moment everything in the Universe came into being. The true origin of these weapons are a mystery to even most Eternals, mainly to those who don't know the truth behind the cosmology of the cosmos.
All Eternity Swords have a specific Rank that they belong to, with those Ranks going from Rank 0 to Rank 9, from Rank 0 being the strongest to Rank 9 being the weakest. The stronger the Eternity Sword, the greater their will, leading most Eternity Swords from Rank 4 and above to have their own established personality, whereas Eternity Swords of Ranks 5 and below tend to be more comparable to "blades that run off instinct," as Yuuto's "Wisdom" describes his previous Eternity Sword of Rank 4, "Desire."
While the gap between Ranks 5 to 9 are not extreme, this is not the case for Eternity Swords from Ranks 4 to 0 -- their gap is astronomically large in comparison, although these rankings are purely denoted by the pure power that they possess. It is still capable for a Rank 3 Eternity Sword to best a Rank 2 Eternity Sword, however the chances of such border upon their wielder's experience entirely. Entities that obtain Eternity Swords from Ranks 3 to 1 are "eligible" to become beings of practically unlimited power dubbed "Eternals."
Every Eternity Sword has a name that they know of at Birth, different from their wielder's name. Eternity Swords Ranks 4 and above tend to have a name that harbors over an abstract concept, such as Yuuto's "Wisdom," or Shun's "World."
Those in possession of Eternity Swords normally are so for differing reasons. Most of the time, a "Contract" is formed between the Wielder and Sword, such as Yuuto and "Desire." However, there is also the case of an Eternity Sword simply overtaking one's mind and body, such as Shun's "Oath." Aside from this method, the only other way to have an Eternity Sword is to have been a god in one's previous life. Examples of this are in Seinarukana, with some of the main characters, such as Nozomu and Nozomi, being reincarnations of gods of another world.
All Eternity Swords have the ability to manipulate Mana, the fundamental energy that is, for all intents and purposes, the life of a planet. That aside, they generally vary in what specific element(s) or abilities that they have. For Example, Kouin's "Karma" cannot manipulate any element, however it boosts Kouin's inherent abilities to an unsettling degree, whereas others, such as Tokimi's "Prophecy" allows her to see and manipulate Time itself, and Rogas' "Fate" allows him to know the Fate of all things.
Shugo Shinjuu, (守護神獣), - The avatar of an Eternity Sword's will. Rank 3 and higher can manifest a physical form that represents the will. Although, within the Time Tree, it's apparently possible for Eternity Swords Ranks 4-6 to physically manifest them if their owners are/were a god.
Despite having the name "Eternity Sword," these swords do not have to come in the shape of weaponry. Some of them come in the shapes of rings or whips. Generally, Eternity Swords look like and are, for all practical reasons, Swords, although exceptions certainly exist.
Original form: unknown. Broken form: tiny fly-like violet light . Current form: two shining bracelets (using the shattered Kuukyo as a container), one on each wrist, but form is not fixed thus Kyouko can manipulate it at will.