Dr. Gabriel Valack was a prisoner inside a reinforced "bullet-resistant polycarbonate" cell at Eichen House until he escaped and took over the facility in Season 5.
Most of Season 5 is a series of flashbacks revealed during an interview between Valack and Lydia as he struggles to help her remember what happened leading up to a final confrontation between her friends and the The Dread Doctors.
Dr. Valack was played by actor Steven Brand.
He was once Chief Medical Officer at Eichen House. He performed trepanation on supernatural creatures and on at least one Banshee to amplify their abilities. The process worked initially but the increase in power eventually killed them all. He was eventually stopped and locked up himself.
Valack had, via trepanning, a hole in the center of his own skull which gave him a third eye and enhanced abilities. These abilities include projecting visions into people which would render them unconscious, and apparently the ability to appear as other people as he appeared as Conrad Fenris when picking up Lydia from the hospital and seemed to mimic Aiden's appearance while questioning her. The latter ability seemed to work for him even after the Dread Doctors removed his third eye.
When he escaped his cell, he attempted to amplify Lydia Martin's abilities in order to stop the Dread Doctors. When Theo Raeken and his pack attack the facility, Valack pulls out a mask similar to the Dread Doctors' with the intent to put in on Lydia to find out who the final chimera is. Before he could put it on, however, one of Lydia's screams shatters a piece of his skull.
At some point, prior to his incarceration, Valack wrote a book entitled The Dread Doctors. It details the activities of a group of scientists who meld science with the supernatural. Valack used the pen name T.R. McCammon to protect his "reputation." He claims he wrote the book to help people remember their encounters with the Dread Doctors.
Valack takes on the appearance of Aiden to stop Lydia's escape and return her to a cell at Eichen House. He questions her about what happened to her friends at the start of senior year and decides to use Trepanning, drilling a hole in her skull, to help her remember. (Read More...)
The Dread Doctors break into Eichen House and steal Valack's third eye. Using a recorded Banshee scream, Valack managed to crack the walls of his cell. (Read More...)
As he continues to question Lydia, Theo and his pack break in and attempt to take her. (Read More...)