"Chinga," is set in King's native Maine. Seeing Mulder and Scully head to Castle Rock would've been pretty great. Alas, fans didn't get to see Mulder much at all in "Chinga," as the story focused almost entirely on Scully.
What many fans might not be aware of though is that the final version of "Chinga" in many ways departs from King's original script, including having Mulder and Scully be separated for the duration.
In "Chinga," Scully is on vacation in Maine, and ends up getting drawn into a strange case involving a mother, her daughter, and the titular doll. The mom, Melissa, keeps getting visions of people's horrific self-inflicted deaths before they happen, with each death seemingly being caused by Chinga after the victim does something to anger Melissa's daughter Polly. Many in the town suspect Melissa of witchcraft, but it's the doll that's evil, and its reign of terror finally ends when Scully manages to cook it inside a microwave.
It's unclear just how much of King's script actually made it to the screen. King had met X-Files star David Duchovny while they were doing Celebrity Jeopardy together, and after expressing both his fandom and desire to write for the show, Duchovny got King in touch with Chris Carter. King ended up writing "Chinga" at his home, then sending it in. However, Carter wasn't happy with a lot of King's script, especially exchanges between Mulder and Scully that he didn't feel fit the characters. So Carter rewrote quite a bit of King's material, leading to them sharing writing credit on "Chinga." One element Carter brought into the story was its exclusion of Mulder.
Via Screen Rant