The Cardassian who tortures Picard in "Chain of Command" is Gul Madred. He interrogates Picard after capturing him during a covert Starfleet mission on Celtris III.
Madred uses physical abuse, sensory deprivation, dehydration and psychological manipulation to force Picard to reveal Federation plans for Minos Korva. His most famous tactic is insisting that Picard see five lights when there are actually four.
The character was played by David Warner, whose restrained performance made Madred one of The Next Generation's most memorable antagonists. Madred is sometimes confused with other Cardassian officers, including Gul Dukat, a major Deep Space Nine character, and Gul Evek, who appears in several Star Trek episodes.
Picard ultimately resists Madred's final attempt to break him. As he is released, he shouts, "There are four lights!" The episode is a two-part sixth-season story that originally aired in December 1992.