The Letheans were a telepathic humanoid species. They were skilled at performing telepathic attacks which almost always proved fatal to the victim. Because of this skill, criminals often hired them. (DS9 episode: "The Sword of Kahless")
In 2371, a Lethean named Altovar traveled to Deep Space 9 and attempted to acquire a few hundred millimeters of bio-mimetic gel from Doctor Julian Bashir. After Bashir refused his request, Altovar telepathically attacked him and left him in a deep coma. Subsequently, Bashir recovered and Altovar was arrested by Constable Odo before he managed to leave the station. (DS9 episode: "Distant Voices")
- The Letheans were also namechecked in The Art of the Impossible in a list of telepathic species which included the Vulcans and Betazoids.
In 2391, during the Klingon-Gorn War, King Slathis of the Gorn Hegemony attempted to acquire the services of Lethean mercenaries to battle the Klingon Empire. Outside analysts doubted he had the resources to pay both them and the Nausicaans he had already hired. (ST website: The Path to 2409)
In 2401 what appeared to be Klingon Councilor Konjah was attacked by Lethean mercenaries on his way home after a night of revelry. The telepathic assault caused the Undine masquerading as Konjah to lose his humanoid form, and the Letheans quickly killed him and presented his body to the Klingon High Council. The House of Konjah was stripped of its lands and titles, and in an ensuing witch hunt all its members were killed or forced into hiding. (ST website: The Path to 2409)
By 2409 many Letheans fought for the Klingon Defense Force, having offered their services to Chancellor J'mpok. They were not granted full citizenship, however, and those who did not serve in the KDF often eked out a miserable existence in slums on the fringes of First City on Qo'noS. Some Letheans of this era were known to have telekinetic abilities in addition to their other powers. (ST video game: Star Trek Online, STO mission: "Off the Beaten Path")