Godchild by Kaori Yuki

Deep in the heart of nineteenth-century London, a young man named Cain emerges from the shadowy cobblestone streets. Forced to become an Earl upon the untimely death of his father, Cain assumes the role of head of the Hargreves, an aristocratic family with a dark past that seems to unfold one scandalous revelation at a time.

Lost, lonely and disenchanted with his fate, Cain develops an obsession with lethal poisons and an uncanny ability to solve the mysterious murders that seem to follow him wherever he goes.

With the aid of Riff, his faithful manservant who has a background in medicine, and Oscar, his long-time friend, Cain wades through his tormented life and struggles to find the truth behind his father's mysterious death and alleged involvement with a sinister secret society.