The Unclean are an ancient species whose origins are not truly clear but it is known that they existed within the Alpha Quadrant during the reign of the Furies. They were the most hated enemy of the Furies who were the undisputed rulers of known space, who had enslaved the ancestors of many of the current races which include the Vulcans, Klingons and Humans. The Furies attempted at combating this menace by using their ultimate weapon; the fear generating device which they cowed their slaves with but the insectoid Unclean were unaffected due to their 'hive'-like nature. Gradually, the Furies fell before the swarms of viroids and the demonic foes resorted to using their slaves against the Unclean which failed. Eventually, the Furies were defeated but in the final battle, the Unclean vanished for thousands of years.
The Unclean are a non-humanoid, six limbed insectoid, vacuum resistant, viroid species which seek out new sources of DNA and devour it as well as incorporate it within their genetic matrix. This essentially allows them to evolve quickly and rapidly. They are also capable of discerning knowledge from those they have incorporated into themselves. They are capable of merging into a single "mass" that effectively serves as a supercomputer allowing the many DNA strains to be linked as well as linking the minds they have absorbed. This provides them with an amazing level of computational power.
Despite their ability to devour and incorporate the DNA of their victims, strong and powerful minds are capable of actually retaining their identity as well as capable of taking control of individual members of the viroids though this is rare. The crew of Deep Space 9 were able to defeat a surviving "segment" of Unclean in this time as they attempted to disrupt the Bajoran wormhole and travel back to the original battle with the Furies by deliberately "feeding" them a temporally-displaced version of the Dax symbiont while they were on board a Jem'Hadar satellite, the symbiont connecting with enough independent minds in the Unclean collective to recreate a mouth and issue the station's self-destruct sequence. (DS9 - Invasion! novel: Time's Enemy)