While under the command of Captain Darson, the Adelphi was assigned to a first contact mission to the planet Ghorusda. The mission was centered around civilian specialist Tam Elbrun, an unusually talented Betazoid telepath, who joined the mission to facilitate contact with the culturally complex Ghorusdans. The situation was considered extremely delicate and volatile, and due to a misunderstanding, a conflict broke out.
In the incident, later known as the Ghorusda Disaster, forty-seven Adelphi crewmembers were killed, including Captain Darson, and two friends of Riker from his class at Starfleet Academy. Later, a Starfleet board of inquiry blamed Darson for "carelessness" concerning Ghorusdan cultural taboos. (TNG: "Tin Man")
Background information Edit
The Adelphi was classified as an Ambassador-class heavy cruiser with the registry NCC-26849 in an internal list of starships, dated September 26, 1990.  This information about the ship later appeared in the Star Trek Encyclopedia (4th ed., vol. 1, p. 10).