Spinal Parasite

The Spinal Parasite was a "monster of the week" featured in the X-Files episode "Roadrunners" - it is a mysterious entity of unknown origins, named after the fact it requires a human host to properly function and controls them via attaching itself to their spine - it was served by a religious cult that believed it to be the Seconding Coming of Christ and were willing to murder on its behalf.

History
the Spinal Parasite is an enigmatic creature whose origins are unexplored, it could be an earth-borne mutation, genetic experiment, hidden species or even an extra-terrestrial entity but what is known is that at some point it gathered a literal cult following, who aided the creature in finding new hosts by brutally murdering former hosts via stoning.

Such a fate befell a disabled man, whom the unfortunate Hank Gulatarski happened upon, the cult proceeded to remove the Spinal Parasite from the disabled man's corpse and forced Hank to become its new host.

The brutal murder attracted the attention of Dana Scully, whom the cult tried to trick into service by asking her to treat Hank - however in the process of investigating the events Dana became aware of the true nature of the cult and the Spinal Parasite compels Hank to willingly give himself to said cult, who restrain a hapless Scully and stone Hank to death in front of her eyes.

The cult then proceed to infect Scully with the Spinal Parasite, thankfully she is rescued by Agent Doggett, who manages to remove the Spinal Parasite without killing Dana and shoots the creature dead, ending its influence over the world, the cult are devastated by its death and do not resist being arrested, their only defence against said imprisonment being "religious persecution".