Letheans are a telepathic species. Their telepathic attacks are usually always fatal. Their skills led them to being hired by various criminal groups in the 24th century. They are known for being incredibly stubborn.
In 2371, a Lethean named Altovar, attempted to acquire a few hundred milliliters of bio-memetic gel from Julian Bashir. After Bashir refused to give him the gel, Altovar telepathically attacked him, placing him in a coma. (DS9: "Distant Voices")
A year later, Toral, the illegitimate son of Duras, hired another Lethean, Soto, to read Kor's mind, after the Dahar Master discovered the location of the mythical Sword of Kahless. Soto travelled with Toral to the Gamma Quadrant. (DS9: "The Sword of Kahless")