The Hihiirokane was held in Jubei's clan deep within Japan for generations. In the beginning of the Dark War, this village was burnt down in the nuclear strikes sent by each country to try and annihilate the Black Beast and Hihiirokane was moved to a new village with the surviving members of the clan. Tomonori, one of the clan's members, took the sword with Jubei's blessing to kill Kazuma Kval and Yuuki Terumi before they would merge.  He was too late and battled them, landing a severe blow to Kazuma's stomach, but it was not enough to stop him from being murdered by the duo with their variant of the Geminus Anguium: Ouroboros.  The sword was recovered by Valkenhayn R. Hellsing and eventually Jubei borrowed it from Rachel Alucard so he could use it to create an opening for Celica A. Mercury to enter the Black Beast. After the Dark War, it was buried with Clavis Alucard upon his death - a spell named Prototype Absolute Defence: Blade of Reflections was placed over the coffin to prevent others from accessing the weapon.
Over 80 years later, Terumi ransacked Clavis' coffin and fought against the spell, winning and taking Hihiirokane.  Terumi absorbed Hihiirokane into himself temporarily and used it to permanently sever his connection to Hazama and later fused it with the real Ouroboros.  He attacked Hakumen, shattering his soul with the Hihiirokane's properties.  Terumi later fought against Ragna the Bloodedge for the Azure using the Hihiirokane-Ouroboros hybrid, but was defeated, with both the sword and Nox being lost within the deepest part of the Boundary. Platinum the Trinity, using the Sekkigan with the Arma Reboare: Muchourin, created a replica of Hihiirokane that was used by Jin Kisaragi to slash down Terumi before his death. 
The most interesting aspect about Hihiirokane is its ability to cut a soul. When being used, it emits a golden glow, and can transform its shape into various forms. One form is that of a large sword with a massive claw on its end. Usually, Hihiirokane takes the form of a brown sword with multiple red lines running down its handle and brown blade. It was once concealed in a black scabbard within a paulownia wood box during its time in the village. It could take other forms, such as a golden cube and as the snake-head on the end of Ouroboros.
BlazBlue: Central Fiction Library Entry
- Time Killing Blade
A sword passed down among Jubei's clan. It's unknown who smelted it and for what goal, but it's said to be capable of attacking not only the body, but also the soul. It can be thought of a weapon capable of fighting those with a 'spiritual body'. 
Hihi'irokane (緋々色金) is a mythical Japanese alloy that is said to glow in a way that resembles flames. The name literally translates to "brilliant scarlet metal." Parts of the Susano'o Unit and Murakumo Unit's armor are made out of this mythical alloy.
The sword itself is written with the characters 刻殺しの刀 tokigoroshi no katana, literally "time-killing blade," but is given the pronunciation of ヒヒイロカネ hihiirokane. The character for "time" used here, 刻 koku, is the kanji for the traditional Chinese unit of time equating approximately two hours, and is used as an alternative to the more common character for time, 時 toki.
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