From accounts of Arthur Rimbaud, Arahabaki appeared as a quadrupedal, black beast, with fur, a tail, and eyes all like fire and black smoke. After Rimbaud used it as the base of Illuminations, it gained a form resembling the Port Mafia's former boss.
Arahabaki had no single concept of anything other than its own existence. As such, it had no solid personality, only existing as, essentially, the personification of calamity and destruction. Its instincts told it that it'd been sealed away, but that's where its awareness ended. 
Over a decade prior to the main events, Arahabaki existed as a god of calamity sealed away in a facility of the Japanese government. How the government obtained and sealed Arahabaki is unknown, but they used it as part of a series of experiments using abilities and otherworldly entities together - ultimately creating artificial abilities.
Arthur Rimbaud infiltrated the facility, attempting to steal Arahabaki. He failed, only removing its seal. Without the seal, Arahabaki was freed and destroyed the facility, forming the mysterious Cone Street.
Between Illuminations' interference and the seal, Arahabaki ceased to exist as a calamitous god, now harboring its power and existence inside Chūya.
Powers & Abilities
- Destruction Embodiment:Arahabaki existed solely for pure destruction, a mass of anything that could destroy and devastate all around it indiscriminately.
- Corruption (汚濁, Ojoku?): As part of Chūya, Arabahaki generally remained 'dormant' inside him at all times, stabilized by his mere existence. However, when Chūya activates Corruption, he is overrun by his ability, now amplified Arahabaki. True to its name, Chūya loses all awareness, merely taking extreme pleasure in destroying everything around him. 
- Arahabaki is an ancient Japanese god shrouded in mystery, its origins and historical relevance drenched with unreliable, scattered accounts. One theory considered it a sort of "guest god", on or lower a level of power of the main of a shrine it manifests in.