Cal Jeppy

Cal Jeppy is the true villain of the X-File's episode "Invocation" and is one of relatively few villains in the show's long history who is neither a monster, alien or other paranomal entity but rather a malicious human, this in some ways makes him one of the more "realistic" villains on the show and likewise one of the more unnerving.

History
Cal was an abusive man who forced the son of his girlfriend to drug a young boy named Billy Underwood so that he could abduct him at a school fair - after abducting the boy he proceeded to murder him and bury him in a shallow grave.

For many years Cal continued his criminal lifestyle and had plans to abduct and kill Josh Underwood, yet his evil plans were foiled when Billy literally rose from the dead as a ghost to protect his brother and avenge his own death - ultimately leading Mulder and Doggett to Cal and the shallow grave where Billy's now skeletal remains lay.

Cal was arrested for the abduction and murder of Billy, likely also facing charges for his attempt on Josh and considering his nature there is a strong possibility he murdered other children as well : while his fate is ultimately unknown given the sheer evidence against him and the fact Ronal Purnell was willing to confess to drugging Billy on Cal's behalf it is almost certain he will serve a long prison sentence for his crimes.