Erik Pressman was a treacherous Starfleet Rear Admiral who violated a treaty with the Romulans the Federation made in good faith. He's the main antagonist in Star Trek: The Next Generation episode "The Pegasus." He was portrayed by Terry O'Quinn.
In 2358, as captain of the USS Pegasus, Pressman was placed in charge of testing an experimental phasing cloaking device, which had been developed by a secret group at Starfleet Security in direct violation of the Treaty of Algeron, which banned the use of cloaking technology aboard Federation starships. In response to this illegal activity, the crew of the Pegasus mutinied against Pressman and attempted to gain control of the ship. Pressman and a small group of officers, including a young Ensign William T. Riker, were able to escape the ship, and observed what appeared to be the destruction of the ship from an escape pod.
Attempt to retrieve the Pegasus
Twelve years later, Pressman, now an admiral with Starfleet Intelligence, boarded the USS Enterprise-D to mount a mission to salvage the Pegasus. Having learned from a highly-placed operative in the Romulan High Command that the Romulans had become aware of the Pegasus's experiment, Pressman was determined to find it before the Romulans. Reuniting with Riker, who by this point was first officer of the Enterprise-D, Pressman expressed his desire to continue the illegal experiments. Feeling unable to continue his role in the conspiracy, Riker revealed the true nature of Pressman's mission to Captain Picard, who promptly ordered Pressman placed under arrest. Although facing a court martial for his role in the illegal activities, Pressman informed Picard that he had "a lot of friends" at Starfleet Command.