The Black Monk - Anton Chekhov

Aspiring academic Andrei Kovrin, while summering in the countryside per the advice of a physician, is haunted by the apparition of a black monk that appears only to him and encourages him in his intellectual pursuits. Although Kovrin is the only one who can see the apparition, the monk assures him that, even if he were a creation of the imagination, he would still be a thing of nature and consequently real. Chekhov uses this vehicle for a gothic exploration into scholarly obsession and madness. - Summary by Daniel Davison