A piece of Federation equipment taken by the Ferengi from an unmanned outpost on Gamma Tauri IV, the theft that leads the Enterprise-D into Starfleet's first face-to-face meeting with them.
The T-9 energy converter is the object of the chase that opens the Next Generation episode The Last Outpost. A Ferengi vessel steals it from an unmanned Federation monitor post on Gamma Tauri IV, and the Enterprise-D pursues the thieves to the Delphi Ardu system.
There both ships are held fast and drained of power by a force coming from the planet, an outpost of the long-dead Tkon Empire. Each crew at first blames the other, and teams from both beam down, where the Ferengi under Letek ambush Riker's party with energy whips.
After Riker satisfies the outpost's guardian, Portal, and both ships are freed, the Ferengi DaiMon Tarr agrees to return the converter.