The Letheans were a telepathic species whose telepathic attacks were almost always fatal. Their skills led them to being hired by various crime groups in the 24th century. They were known for being incredibly stubborn.
In 2371, a Lethean named Altovar attempted to acquire a few hundred milliliters of bio-mimetic gel from Julian Bashir. After Bashir refused to give him the gel, Altovar telepathically attacked him, placing him in a coma. Altovar didn't get very far until he was arrested by Odo. (DS9: "Distant Voices")
A year later, Toral, the illegitimate son of Duras, hired a Lethean named Soto to read Kor's mind after the Dahar Master discovered the location of the mythical Sword of Kahless. (DS9: "The Sword of Kahless")
Michael Westmore found the physical design of a Lethean was "really interesting," owing to it looking unlike anything previously depicted on Star Trek: Deep Space Nine. This was despite the fact that the Lethean involved recycled makeup. "Because of last-minute casting, I didn't have time for sketches," he explained, "so the headpiece is converted from another alien. I would have needed about three days to build a mold of his entire head, and I didn't have three days. I had one sculptor doing the hands, one doing a separate thumb on the hands, another doing the face, and a fourth doing the lower part of the face on a separate mold. The rubber literally was coming out of the oven and going right to the set." (Star Trek: Deep Space Nine Companion (p. 223)) Westmore further discusses the creation of the Lethean makeup in the "Michael Westmore's Aliens, Season Four" special feature on the DS9 Season 4 DVD.
Letheans figure prominently in the PC game Klingon Honor Guard as one of the more formidable adversaries faced by the player, in the role of an Honor Guard trainee. In the game, the Letheans are always at least partially cloaked, and only become visible to the player at the moment they attack, or when they are killed or wounded. The Letheans attack using their telepathic abilities which are illustrated during game play by sparks of lightning-like energy, or, during an intense attack, a flash resembling ball lightning. In the game play manual, the name of the species is spelled "Lethians".
Letheans are also a playable race in Star Trek Online in the service of the Klingon Empire. They have access to a unique character trait called "Rapture", whereby they deal psionic damage over time with a chance to stun their target.