The upcoming episode of "Criminal Minds" season 12 will see the Behavioral Analysis Unit tracking down a serial killer in the woods. 

Titled "Keeper," the synopsis of the fourth episode reveals that the BAU, now headed by Emily Prentiss (Paget Brewster), will be dealing with a monster of a serial killer. As an ode to Halloween, reports tease that the killer on the loose may be a supernatural being. However, that remains to be seen.

The BAU will look for the killer along the Appalachian Trail in rural Virginia and take him down before he kills again. He will apparently have killed seven people already, all being tortured prior to finishing them off in a painful way. 

A promo trailer has also been released for the new episode and it teases the BAU team searching for the killer. "What is going on?" Emily asks, as a voice-over teases an episode "that will chill you to the bone." A dirty hand with sharp nails can be seen holding onto a tree trunk and the clip teases the existence of a supernatural creature. "This is scary," Penelope (Kirsten Vangsness) says.

The previous episode saw the departure of Thomas Gibson's Aaron Hotchner as he was tasked with a temporary assignment. Gibson's character being written off was the result of the actor's dispute with a writer on the show. Brewster's Emily Prentiss, who has been a beloved character on the show since season 2, returned to replace him. Prentiss' involvement with the BAU has been on and off for several seasons, but it looks like she's back for good. During her absence, it has been said that Prentiss has been in charge of Interpol.

"Criminal Minds" season 12 did not air an episode this week, due to the Presidential Debate. The show will return on Oct. 26, Wednesday, at 9/8c on CBS.