Gabriel Valack, before the series timeline, was chief medical examiner of Eichen House. He performed inhumane, amoral experimentation on supernatural creatures that resulted in many deaths. Valack also became an enemy of the Dread Doctors. For his heinous experimentation, Valack was imprisoned in Eichen House, becoming a long-term prisoner, serving time in the underground prison for supernatural beings.
A prisoner in the Eichen House supernatural ward, Dr. Gabriel Valack is kept largely isolated from other people because he can turn them into vegetables. He drilled a hole in his head to open the access to his third eye. While usually metaphorical, in his case it happens to be literal. A third eyeball in his head opens, allowing him to enter other people's minds. When he removes the wrapping over the hole, others seem to drawn to look inside.
Valack is given romance novels to read.
His cell is compartmentalized so there is a plexiglass wall between his living space and the door outside Unit 8.
Alan Deaton visits him when he wants to know more about South American mythology and what's happening to Derek. Valack gives him a vision of La Iglesia.
Peter is later imprisoned with Valack, although there aren't two bunks in the room so it's unclear if this is a permanent arrangement or specific punishment.
In 5x05 - A Novel Approach, Stiles and Lydia visit Valack to talk about The Dread Doctors, a book he wrote. He used to have Peter a cellmate, but Peter is gone. Valack wrote the book because no one believed him.