Douwd

The Douwd were a powerful, immortal race of energy beings. They possessed near omnipotent powers, including the power to wipe out an entire species from existance. Some Douwd impersonated members of other species.

In 2366, the Douwd known as Kevin Uxbridge was encountered on planet Delta Rana IV, three days after the destruction of the colony by the Husnock. Uxbridge was left in self-imposed isolation on the planet after wiping out the entire Husnock civilization in retaliation. (TNG episode: "The Survivors")

Other References

 * TNG novel: Q-Space
 * TNG comic: Planet X