Question: when did mulder return to x files?
Answer: In 8x14 "This is Not Happening!", Scully has a nightmare of Mulder lying dead aboard the alien craft, only to discover at the end of the episode, when he is returned, he really is dead. In 8x15 "Dead Alive" and 8x16 "Three Words", Mulder is now alive and is recovering from his abduction and aftermath.