Husnock

The Husnock are an alien of the week blamed for attacking a Federation colony on Rana IV. In the aftermath of that attack they are extinct, if they ever actually existed.

Dubious Existence

The only evidence for the existence of the Husnock race is the word of the Douwd, who claimed to have exterminated all of them after his human wife was killed in their attack on Rana IV. He claimed to have completely wiped them out -- all 50 billion of them -- and no remains of their civilization have ever appeared. It is entirely possible that they are a made-up scape-goat.

Attributes

The Douwd described the Husnock as a race of "hideous intelligence, who new only aggression... destruction." Even assuming they actually existed, this is a questionable description, as it seems unlikely that a race with such behavior could build a thriving civilization.

The Douwd created an illusionary Husnock starship to attack the Enterprise-D when it came to investigate a distress signal from the Rana IV colony. This ship had five times the mass of the Enterprise and carried "enough firepower to pulverize a planet". If the Husnock were real, and this ship were representative of their capabilities, they would have been one of the most powerful and dangerous civilizations in the Alpha Quadrant.