Gul Madred was a Cardassian officer who captured and tortured Jean-Luc Picard in 2369, trying to break his will with the trick of the four lights.
Gul Madred was a Cardassian interrogator. In 2369 Picard led a secret mission to Celtris III with Beverly Crusher and Worf to find a reported Cardassian weapons project. The report was a trap designed to capture Picard, and while Crusher and Worf escaped, Picard was taken prisoner and handed over to Madred.
Madred used drugs, pain and humiliation to break Picard, trying to strip away his identity. His main tool was a set of four lights, which he insisted Picard must say were five. Picard refused despite days of torture, and in quiet moments Madred revealed a hard childhood on the streets of Cardassia. Picard was freed when the Enterprise under Captain Edward Jellico forced the Cardassians to back down in a border standoff. Picard later admitted to Deanna Troi that by the end he had seen five lights.
The two-part story Chain of Command is one of the most powerful in Star Trek, a study of torture and resistance inspired in part by real accounts.