Vahni Vahltupali were tall, slender and vaguely humanoid, having two legs, a narrow torso, two tentacles in place of arms, and a bulbous head-like projection ringed by a complex ocular organ. They possessed neither vocal nor auditory organs, but were able to change the colour and texture of their flesh akin to an Earth octopus. Their skeleton was firm but malleable, and many-jointed, enabling dramatic contortions.
The Vahni communicated by flashing different patterns of colour and texture across their epidermis.
Whorls and eddies of white were the equivalent of screams of terror. A darkening of the skin to deep indigo indicated sorrow or grief.
As of 2376, the population of the Vahni homeworld was approximately four billion. They possessed a peaceful and benevolent global government.
A typical Vahni city was laid out in concentric circles, its colourful metal and glass buildings usually only one or two stories tall. Pedestrian thoroughfares, public meeting areas and numerous greenswards were interspersed throughout. Underground transportation grids allowed fast and easy access to local and distant points. Commander Elias Vaughn found the city he visited to be breathtakingly beautiful.
They were accomplished astronomers and physicists, had explored their solar system, discovered subspace and utilised subspace communication, and were on the brink of developing both transporter technology and warp drive.
In the 22nd century, the Vahni homeworld began to be affected by earthquakes. These quakes occurred with no warning but caused little damage and happened decades apart. Over the years, the length of the intervals between them decreased and their destructive power increased. The most powerful quake occurred in 2375.
In June 2376, the Vahni made first contact with the Federation starship USS Defiant. Just after the Defiant's arrival, the Vahni experienced the most powerful earthquake to date. This was coupled with the destruction of their moon. (DS9 - Mission Gamma novel: Twilight)