The stand-off at Delphi Ardu in which the Enterprise-D and a Ferengi vessel were both held powerless by an ancient Tkon outpost, giving Starfleet its first direct look at the Ferengi.
In The Next Generation episode The Last Outpost, the Enterprise-D chases a Ferengi ship that has stolen a T-9 energy converter from Gamma Tauri IV. Near the planet Delphi Ardu both ships lose power, and each assumes the other is responsible until Picard and DaiMon Tarr work out that the planet is the cause.
Away teams from both sides beam down. The Ferengi, led by Letek, ambush Riker's party with energy whips, the first time Starfleet officers have seen the species in person.
The planet proves to be an outpost of the long-dead Tkon Empire, guarded by Portal 63, who wakes to judge the intruders. Riker answers his challenge calmly, drawing on Sun Tzu, and wins his respect. The Portal releases both ships, and Riker asks that the Ferengi be spared so that they may learn.