The Beast Spear is a supernatural weapon crafted and forged for the purpose of slaying Youkai.
It was created over 2000 years before the start of the story by Giryou, the son of a master blade-maker, after his sister Jie Mei sacrificed herself to make the spear's blade. Driven mad by his sister's sacrifice, Giryou crafted the Beast Spear, pouring all of his hatred and desire for revenge against Hakumen no Mono, the Youkai responsible for the death of his parents, and by indirect extent, his sister. His very body became the spear's shaft.
With the completion of the weapon, the Beast Spear searched the world for Hakumen no Mono, slaying thousands of youkai in its relentless search to satsify Giryou's ruthless desire for revenge. Finally, hundreds upon hundreds of youkai banded together, transforming into a single long red sash that bound the Beast Spear to a valley. It is unknown how long it remained there but at some point it was released by a man named Shagakusha who became its first wielder and eventually it transformed him into the first Azafuse who came to be known as Nagatobimaru. The Beast Spear was wielded by many over the centuries: by a farmer to protect his family, by a woman to avenge her lover, by a mercenary who killed for money, until finally it came into the possession of the samurai warrior who fought against Tora, then called Nagatobimaru, and impaled him to a stone with the Beast Spear. There it remained keeping Tora imprisoned as the samurai's family, the Aotsuki clan, built a shrine and temple around Tora to insure his imprisonment. 500 years later, a boy named Ushio Aotsuki, who was a descendant of the warrior, found the cellar where Tora and the Beast Spear were, and was finally forced to release him to save his friends, therefore coming into possession of the Beast Spear himself.
It is later revealed that there have been many wielders of the Beast Spear before Ushio and his ancestor who sealed Tora. All these warriors across the centuries wielded the Beast Spear, but after wielding the weapon for so long they each in turn transformed into Youkai very similar to Tora, with the same powers and appearance albeit having distinctive characteristics (different faces, claws or feet, different markings, different kinds of hair, etc.), and they were all called the Azafuse. These Youkai all turned to stone to await the final battle with Hakumen no Mono as the Beast Spear continued to change hands over the centuries.
A revelation later shows that the Azafuse are not the final transformation by prolonged use of Beast Spear. The Azafuse have only a single desire, transmitted via the Beast Spear: Their hatred of Hakumen no Mono. In an ironic twist of fate, this prolonged hatred eventually transformed all Azafuse into the appearance of Hakumen no Mono. Thus the Beast Spear actually ultimately gives birth to more Hakumen no Mono if left unchecked. There are only two Azafuse known in the series not to become Hakumen no Mono: Guren and Tora. Guren desired to kill both humans and youkai, thus overcame the Beast Spear's hatred for Hakumen no Mono. Tora was affected by Ushio's personality and overcame his hatred as well. The remaining Azafuse that transform into the appearance of Hakumen no Mono lamented for not knowing a person like Ushio in their lifetime, just as Tora had.
It is also important to note that the Beast Spear has a distinct aura that Hakumen no Mono can detect, which is a double edged sword for the Beast Spear wielders. The Beast Spear can vanquish Hakumen no Mono minions, preventing more tragedy. However, the wielder would have to sacrifice more of their souls as more and more foes come after them. There are 2 instances where Hakumen no Mono cannot detect the Beast Spear aura. First, the Red Cloth created by numerous Bakemono who sacrifice themselves to seal the Beast Spear can mask its presence. The amount of Red Cloth tied to the Beast Spear is proportional to the strength or aura that the Beast Spear can unleash. Secondly, the Beast Spear can be concealed by a youkai stabbing it into them. Only Tora was able to withstand the piercing of the spear and survived but was pinned for 500 years as a result.