Cardinal Thread

When the "red thread" that connects people to each other breaks, a strange phenomenon occurs! It's the Taisho Era, and a crisis is quietly creeping towards the imperial capital. Sazuki, a boy who works for his childhood friend's apothecary, has a drunken, sickly, and helpless father. One day, while dreaming of his younger sister, Takamizu, who was sold to a brothel because of their father's debt, Sazuki witnesses an oddly shaped monster— and a pretty girl and a mysterious gentleman who slay it in half. It is said that when a person's heart is twisted, a thread called "kizuna" gets tangled and a deformed creature appears.

(Source: Enterbrain, translated)

Note: Chapter count includes chapter 0.