The Letheans are a telepathic species, whose telepathic attacks are almost 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 the 23rd century, a Lethean flagship, arranged to meet the USS Enterprise for first contact, never arrived. Instead, the flagship was accidentally met with the USS Phoenix-X and the USS Atlantis from the future, ruining their perception of the Federation. During this encounter, a part of the Atlantis` starship technology, a telepathic interface module, had been broken off whilst in mid-battle with the Phoenix-X and was retrieved by the Lethean Captain Flarrd after both future ships disappeared into another time-jump. ("Temporal Doom! Part II")
In the 26th century, thanks to the acquisition of the telepathic interface module in the 23rd century, the Lethean had become "super-telepathic" to the point of telekinesis. When met with the time-traveling Phoenix-X, in this time period, Captain Plarrd, of the Lethean vessel Mikonikoff, was able to force the Phoenix-X crew to their knees by a single thought. However, by this time, Starfleet still had a hold over the Lethean. Whatever technology advances the Lethean had in this century, was by no means comparable to Starfleet. Despite their telekinetic ability, the Lethean feared Starfleet and the Mikonikoff escaped at the sight and approach of their present day adversaries. Before escaping, Plarrd claimed that, by the use of their advanced telepathic power, they were able to summon the Sphere Builders to attack at the Procyon system. ("Temporal Doom! Part II")