A cranial implant was a device used by operatives of the Obsidian Order. It was implanted into a person's skull and was designed to produce large amounts of endorphins if a person was tortured. This would allow a person to be immune to severe pain and experience a pleasurable feeling.
In 2349, Elim Garak had a cranial implant implanted in his skull by Mindur Timot. Garak initially thought the device was being installed due to his low rating for pain he had tested for when he used the enhancer, although Timot assured him all Obsidian Order operatives would soon be implanted with the device that Timot called "the wire". (DS9 novel: A Stitch in Time)
In 2368, after he was exiled to Deep Space 9, Garak created a device which allowed him to trigger the implant at any time, alleviating his depression. In the beginning, he only used for a few minutes everyday, but began to rely on it more often. He later activated it and let it run continously. In 2370, the implant began to break down, causing Garak to suffer bouts of extreme pain. He approached Quark, who he asked to find him a new implant from Cardassia, although Quark was unable to find one.
Bashir later discovered the implant and shut it down, although Garak's condition worsened. Garak later revealed that Enabran Tain had retired to Arawath colony and Bashir travelled there, hoping to find a cure. Tain later acquiesced to Bashir's desire for information on how to save Garak and had the data sent to Deep Space 9. (DS9 episode: "The Wire")