10 X-Files Fates Worse Than Death
2. The Black Oil
The black oil, also known as Purity and "the black cancer", is a major plot point in the wider mythology of The X-Files and 1998's The X-Files Movie. It makes numerous appearances from its proper introduction in the third season episode, Piper Maru until its last original appearance in the eighth season episode, Vienen.
It is theorized to be an alien lifeforce that has inhabited the world for far longer than mankind has, infecting hosts to force them to do its bidding. Those who are not infected by the black oil are, however, irradiated by its host. The black oil also used its various hosts as incubators for alien life and, in the movie, we actually see an alien growing inside of a firefighter following his exposure to the substance. Once it was done with the hosts, the black oil would then slowly (and painfully) drain out of them. Numerous characters have been infected by the black oil, most notably Nicholas Lea's Alex Krycek, Laurie Holden's Marita Covarrubias, and even Mulder and Scully, only to be cured by a vaccine secretly developed by the Russians.
The black oil has got to be one of the most unpleasant fates in the show. Only a select few have survived it and those who did were changed people. Even more grizzly, of course, was its development over time into something scarily similar to the ominous-looking eggs in Ridley Scott's Alien.