Spender got orders from the "Cigarette Smoking Man" to push and eventually get Mulder and Scully fired from the FBI, which he eventually does in "Two Fathers". Later on he reinstates Mulder and Scully to The X-Files but is shot in the head and apparently killed by his father in "One Son".

Who killed Jeffrey Spender?

In 1999 , Spender was shot and apparently killed by his father in "One Son". Three years later it is revealed in "William", that he survived the gunshot, but was subjected to horribly disfiguring experiments at the hand of his father.

Is Agent Scully immortal?

There are countless references to Scully's immortality in the show, but none make the case better than the words of The X-Files' creator, Chris Carter. In 2014, Carter stated on a Reddit AMA that Scully is, in fact, immortal.