A device placed in Elim Garak's brain by the Obsidian Order to flood him with endorphins under torture, which he misused to dull his exile until it began to fail.
The implant is revealed in the Deep Space Nine episode The Wire. Enabran Tain had it fitted so that Garak, as one of his agents, could never be broken by pain: when triggered, it stimulates the brain's pleasure centres.
Living in exile on a station he finds cold, bright and hostile, Garak built a way to switch it on at will, and eventually left it running all the time. After two years the device is breaking down and his body depends on it. He collapses, and Bashir sits with him through a violent withdrawal while Garak tells three incompatible stories about why he was banished.
To save him, Bashir travels to see Tain, who hands over the medical data he needs. Tain explains that he wants Garak to live a long and miserable life.