(covers information from several alternate timelines)
A Chamber of Horrors was a type of attraction that featured horrifying sights and experiences.
In 2266, Doctor Helen Noel assured James T. Kirk that the room housing the neural neutralizer was not a chamber of horrors. (TOS: "Dagger of the Mind")
In 2267, while stuck in the mirror universe, Leonard McCoy described the ISS Enterprise's sickbay as a chamber of horrors. He illustrated this description with the example of two of his assistants betting on how long it would take an injured man to faint due to his pain. (TOS: "Mirror, Mirror")
In 2373, Odo sarcastically suggested that if Mora Pol acquired the infant changeling, he could sell tickets on the Promenade to "Doctor Mora's Chamber of Horrors". (DS9: "The Begotten")
In 2376, Tom Paris questioned Chakotay's admonition to stay calm, referring to the Borg assimilation chamber they were in as a chamber of horrors. (VOY: "Collective")