Diary Of A Madman

Vincent Price stars in the 1963 horror film Diary Of A Madman. From Wikipedia:

Following the funeral of Simon Cordier (Vincent Price), a French magistrate and amateur sculptor, his secret diary is read out by Simon's pastor friend to a group of people gathered around the table, Simon's servants, and a police captain. The diary reveals that Simon has come into contact with a malevolent entity. The invisible yet corporeal being, called a horla is capable of limited psychokinesis and complete mind control. It implied that Cordier's particular horla is one of a whole race of evil beings which devote themselves to driving humans insane.

Cordier first interacts with the horla when he meets a prisoner whom the entity drove to murder four people. The horla possesses the inmate and attempts to kill Cordier, who in self-defense accidentally kills the man. The magistrate inherits the prisoner's troubles as the horla turns its hauntings toward him.

As the horla begins to destroy Cordier's life, he fears he is going mad and seeks help from an alienist, who suggests he take a hobby. Cordier chooses to pick up his old interest in sculpture, meeting a model along the way. The model, Odette Malotte, is already married, but claims to love Cordier and he pledges his love to her in turn. The horla insists the model is not the charming jewel that Cordier sees, but instead a conniving gold-digger, and compels Cordier to treat her as such. This sets up a conflict in Cordier, that he might not be the astute judge of character that his title indicates.