The Black Beast is a monstrous creature responsible for killing half of the world's population before its defeat at the hands of the Six Heroes. According to Jubei, there was no rhyme or reason with the monsters methods; It simply killed for the sake of killing. The results of the monster's existence is noticeable in the current timeframe of the game. In particular, seithr, a dangerous material that the Black Beast exuded from its own body, covered the planet, causing many areas to become inhospitable, and even altering some of the creatures, turning them into violent beasts. A warrior named Bloodedge was responsible for causing the Beast to fall silent for one year, sacrificing his life to do so. This time frame allowed Nine to develop Ars Magus, and, along with the Six Heroes, be able to combat the Black Beast head on. Ragna takes on the Bloodedge name in his honor, as well as inheriting his jacket and weapon from Jubei. These events were called The Dark War, later referred to as the 'First War of Ars Magus'.
The Black Beast is a resulting combination of Ragna and Nu fusing together inside of the Cauldron of Kagutsuchi, Ragna being its body, and Nu the heart. Ragna in particular houses the violent powers of the Black Beast in his body, although how this came to be has not yet been revealed. Hakumen even refers to Ragna as "The Beast" and "The Dark One" instead of his normal name, and he considers it his primary goal to remove Ragna, lest The Black Beast return once again. In one of the endings in Calamity Trigger, Ragna was defeated by Nu at The Sheol Gate, and they were thrown into the cauldron and smelted together to form the mythical monster. This resulted in the Black Beast, along with Jin Kisaragi, being thrown 100 years into the past. Jin would then become Hakumen of the Six Heroes, and take up the fight against the monster.
In Continuum Shift, it is revealed that the Black Beast was originally a failed product of Yuuki Terumi in an attempt to create Kusanagi, the "Sword to Slay the Gods" from the use of the Azure Grimoire. Terumi would take part in defeating the Black Beast, only to turn against the Six Heroes shortly afterwards. It is based on the story of Yamata no Orochi from Japanese mythology, one of the few such references in the series not to be directly named after its inspiration.
In one of Ragna's bad endings in Continuum Shift, after Rachel Alucard wins against Ragna in a fight, she tells him to "Never mistake the power of the Azure Grimoire as your own", Ragna responds by saying he will use his own power to shape his destiny. After Rachel leaves, Ragna's right arm begins to shake violently and seithr starts to generate. Ragna then sees an image of himself, being fully consumed by the darkness of the Grimoire. As quickly as this moment comes, it vanishes, leaving Ragna exhausted. Inside a Cathedral, Ragna encounters Jin Kisaragi, forcing him to use the Azure Grimoire. As Ragna wins, he loses the last of his self control and becomes the Black Beast itself, causing almost all of the planet to be destroyed. As Jubei and Rachel see Ragna off, she tells him that she is sorry, and that none of this was his fault and that she will see him in another world.
In XBlaze Code: Embryo, the Black Beast is referred to as "an avatar of destruction" that has been calmed down by "the priestess of azure" from ancient times. Just like Ragna, Touya Kagari is also shown to have the ability to transform into the Black Beast.
The Black Beast is a mass of black energy shaped like an eight-headed, serpentine beast with numerous tentacles and red eyes. Its body is never shown, although Ragna's use of his Azure Grimoire implies that it also has claws.
Wounds inflicted by the Beast, or it's Remnants, cause seithr poisoning.
The Beast itself is invulnerable to near everything. It withstood the nuclear attacks of every nation with not even scratches on its own hide. It owes this invincibility to the fact that it exists outside of Logic, and can only be harmed by magic, sorcery, and the various forms of Ars Magus, such as the Nox Nyctores. The Beast can also be harmed by those physically outside of Logic, like Valkenhayn R. Hellsing. Even when the Beast was attacked by the Six Heroes and a multitude of different Ars Magus users, such as Ayame Yayoi, it was still able to stand up to the abuse and proved to be an immensely powerful figure.
One of its most troublesome abilities is it's capability to spawn Remnants. As the Beast is constantly surrounded by a myriad of them, with varying battle capabilities, one has to first push through an army of them before even facing the Black Beast's real body.
The Beast is capable of semi-liquidating itself into a mist form and entering the earth in order to move through the underground at an unimaginable speed for an object of such colossal size and mass. This ability is also possessed by its Remnants.
- The Black Beast is more similar to Perfect Chaos because both the Black Beast and Perfect Chaos have numerous tentacles.