As part of a Romulan plan to conquer Vulcan in 2368, the USS Enterprise-D was to be drawn away from the Romulan Neutral Zone by a faked priority 1 distress call from Dulisian IV, claiming a breakdown of the environmental support system. As a Rutian archaeological vessel was the only other ship near Dulisian IV at the time, the Enterprise was prepared to change course. However, the ruse was quickly revealed when Ambassador Spock sent a transmission warning of the invasion. (TNG: "Unification II")
This planet was only mentioned in dialogue.
"duh-LISS-ee-an" was the pronunciation for this planet's name from the script pronunciation guide.