In their natural state, they were between two and three meters in height and they had no apparent eyes, but they possessed an orifice that resembled a mouth on what most humanoids would call their foreheads. They lived out of phase with most other lifeforms with a positive 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.
- See: Unnamed Devidians
Michael Hood was one of the designers and creators of the alien suits. Their glow was later added in post production.
One of the alien suits was sold off on the It's A Wrap! sale and auction on eBay. It consisted of a blue foam rubber body suit, with extremely wrinkled skin and a large cranial area, with the front of the cranial area indented. Although the costume had five fingers, the feet had six toes. 
In Star Trek Online, set forty years after "Time's Arrow", the Devidians return in a five-part feature episode series, where they have been spotted in the re-established Klingon Neutral Zone. Player captains of the Federation (working for Section 31), the Klingon Empire (working for Klingon Intelligence), and the Romulan Republic (working for either or, depending on which faction the player decided to ally with) are sent to investigate. The Devidians' appearance has been changed to make them look more like ghosts, with them hovering above the ground and being partially transparent, and they are led by a leader referred to as the Shrouded Phantasm.
In Star Trek: Coda, set nearly twenty years after "Time's Arrow", the Devidians return in a three-novel trilogy, where they cause a Temporal Apocalypse, destroying not only multiple alternate timelines, but also parallel universes and quantum realities.