Byouki

猫鬼

The creature known as Byoki began his life as a seemingly ordinary house cat during the late Heian Period. At that time, his name was Haimaru, and he belonged to Sana, the daughter of a powerful onmyodo master. Skittish, Haimaru recoiled at the touch of anyone except his mistress, who carried him in her arms almost everywhere she went. The only other person he seemed to mildly tolerate was Mao, who was able to pet the cat when no one else could.
Eventually, Haimaru was abducted by Mokuzu, a simple-minded groundskeeper who was promised great power if he would aid in a nefarious plot to disrupt the school. Mokuzu stole the cat and dumped it into a pit filled with venomous creatures and wild animals. Amazingly, the cat was the sole survivor but was radically changed after devouring the poisons and blood of its vanquished foes. It evolved into the power cat demon Byoki. Breaking free of its bonds, Byoki devoured the sacred texts of the onmyodo temple, gaining knowledge and then attacking the practitioners. Originally intended to be used as a weapon against other schools, Byoki turned against his masters and murdered indiscriminately.
Finally, he faced off against Mao, who managed to decapitate him. Byoki tried to merge himself with Mao to gain more power but failed. Instead, Mao merged with Byoki's severed body while his head fled, going into hiding beneath a sacred stone until the Great Kanto Earthquake of 1923 liberated him. It was then that the beast fought Mao again, spilling his blood into a time-lost Nanoka and passing through the time portal into the present day.