Husnock warships were large starships operated by the Husnock prior to 2366. This type of vessel was much larger than a Galaxy-class starship, capable of being seen from the surface while in orbit, and capable of destroying all life on a planet.
In 2366, a Husnock warship attacked and destroyed a Federation colony on Rana IV. Unknown to the Husnock, an energy lifeform known as a Douwd was also living on the planet in the form of Kevin Uxbridge. In retaliation for the loss of his wife, who died in the Husnock attack, the Douwd used its enormous powers to completely destroy the entire Husnock race.
Following the Husnock attack, the USS Enterprise-D arrived to investigate finding all life destroyed except for a small patch of grass and a house within which lived the Douwd and his wife (who was really an illusion).
As the Enterprise crew grew more suspicious of Kevin, his wife, and the means by which they had survived, a warship appeared in orbit and attempted to drive the Enterprise away. The Enterprise crew initially believed this was the ship that had attacked the planet, but later discovered that the Douwd himself had created the warship to prevent them from learning the truth about his actions. (TNG: "The Survivors")