Moffat tells EW that the finale will leave viewers "hopefully satisfied, and pleased, and a tiny bit devastated. It's pretty emotional stuff. We get quite dark. It's quite a journey."
In concluding the series, "Hell Bent" will also conclude the emotional story arc that began in "Face the Raven," where the Doctor's companion, Clara Oswald, died. Clara made another brief appearance inside the Doctor's head during last week's powerful and atmospheric episode, "Heaven Sent." The episode showcased Peter Capaldi's incredible talent as the Doctor worked through his grief for Clara while working to escape his personal prison.
Moffat says that, while "Heaven Sent" director Rachel Talalay (Tank Girl) will also be directing the finale, the tones of the two episodes couldn't be more different.
"['Heaven Sent'] is one of the strangest, and most atmospheric, ande crepiest episodes that we've done," said the showrunner. "And then, in episode 12, we're in a huge, epic, big-scale Star Wars world. She's got a tremendous range as a director, covering all of that."
Of course, the question on the minds of many fans is whether Jenna Coleman will return for one last appearance as Clara.
"I'm not talking about that, sorry!" Moffat dodged.
The Doctor Who Series 9 finale airs Saturday, Dec. 5 at 9 p.m. ET on BBC America.