Part of the alien lore in The X-Files is that the alien race can only be killed by using a specific weapon and stabbing them at the base of the neck. The weapon is a sort of stiletto which Agent Mulder originally learns of after finding one hidden in a lamp in his summer home. This happens in the season 3 finale episode "Talitha Cumi".
At the end of that episode, the character commonly referred to as the Alien Bounty Hunter shows up to assassinate Jeremiah Smith, another alien of the shape shifting race who has defected from the conspiracy. A fight ensues and Mulder eventually gets the jump on the bounty hunter and stabs him in the base of the neck with the stiletto and escapes.
In the next episode, the season 4 premiere "Herrenvolk", Scully stops to check the pulse of the bounty hunter who has been lying motionless and presumably dead, with toxic blood oozing from his wound. As she does this, the blood suddenly disappears and the bounty hunter wakes up, pulls the weapon out of his neck, and takes Scully hostage.
So... what just happened? All signs pointed to him being dead, so why did he suddenly come back to life? Did the weapon not work? And why not?