Devidians were a vaguely humanoid, shapeshifting species native to Devidia II. Their physical characteristics composed of being between 2 and 3 meters in height; they had no apparent eyes but they possessed an orifice on what most humanoids would call their forehead, that resembled a mouth. They lived out of phase with most other lifeforms with a phase variance of 0.004. They also possessed the ability to travel through time.
Devidians' only useful form of nourishment was Human neural energy. They acquired this energy by draining the subject of their neural energy, killing the subject. They preyed on Humans from 19th century San Francisco using the cholera epidemic at the time as a cover for their murders.
In 2369, the USS Enterprise-D fired modified photon torpedoes on the Devidians' cavern habitat to prevent them from further incursions.
In Star Trek Online the Devidians have found a way to return to the corporeal universe and have begun to feast on other species, most notably the Cardassians - they plan a mass invasion of the corporeal universe in order to drain the neural energy of other lifeforms, leading to a great many deaths and a five-part storyarc.