Purity

The Purity, more commonly referred to as the black oil is an extraterrestrial lifeform similar to a virus, but possessing of sapience and ability to live outside of a host, at least for limited periods of time. As its nickname suggests, the Purity strongly resembles a black oil-like substance, and shares a close symbiotic relationship with the humanoid Colonists (also known as Reticulans or Greys), which need the Purity as their "life essence" and use it to reproduce their kind.

Newborn Colonists grow as endoparasitoidic lifeforms which are brought into the host's body by the Purity. Besides acting as the Colonists' means of reproduction, however, there is strong evidence that the Purity is a different lifeform on its own right, which possesses intelligence and free will. Evidence to this include first of all the simple fact that not all those infected by the Purity go on to become hosts to Colonists: some are simply killed and others are controlled by the Purity for wholly different purposes, and others yet have seemingly managed to live for relatively long periods of time while infected, with no obvious symptoms. Furthermore, those infected by the black oil often develop highly strange abilities, such as being able to emit great amounts of radiation and remain unaffected by it. Yet another clue to the Purity's independence from the Colnists is that a black oil-like substance has been observed squirting from the ammonia-based Parasitic Ice Worms, which are a wholly different sort of creature.

Although this is only speculation; it is reasonable to assume that the Purity represents the real force behind those called the Colonists; as an alien lifeform capable of controlling any kind of organism (from Humans to Reticulans to Shapeshifters to Ice Worms) they might be the ones interested in colonization of the entire universe, with the Reticulans being simply servants to them. This theory is also compatible with the Well-Manicured Man's claim that the black oil was the original inhabitant of Earth, possibly predating all other forms of life. However, it is equally possible that the Purity and the Colonists represent different states of the same lifeform.

Appearances

 * The X-Files (1993 - 2002)
 * s03e15, "Piper Maru"
 * s03e16, "Apocrypha"
 * s04e09, "Tunguska"
 * s04e10, "Terma"
 * s05e13, "Patient X"
 * s05e14, "The Red and the Black"
 * s06e11, "Two Fathers"
 * s06e11, "One Son"
 * s08e16, "Vienen"
 * s09e19, "The Truth, part I"
 * s09e20, "The Truth, part II"
 * The X-Files: Fight the Future (1998)