An upcoming episode of "Criminal Minds" Season 12 will serve as a game changer for Spencer Reid (Matthew Gray Gubler). The FBI's Behavioral Analysis Unit (BAU) will also deal with a case that has darker undertones than usual.

The 13^th episode of "Criminal Minds" Season 12 titled "Spencer" will be an eventful one for Reid, TVLine reported. Kirsten Vangsness, who portrays Penelope Garcia in the long-running CBS crime drama, co-wrote the episode with showrunner Erica Messer.

Vangsness said that she seems to write "Criminal Minds" episodes that "change the course for people," which is true given how she penned the episodes where Gideon (Mandy Patinkin) got killed and Derek (Shemar Moore) left. Vangsness, however, stressed that the episodes do not necessarily mean "the ends of people, it just changes their course," and the Reid-centric one is definitely a "course changer."

"We save lives, and that's what matters. And every life we save is a win." Catch up on #CriminalMinds now: https://t.co/kPP4WDPqiQ pic.twitter.com/6HuQbLV2CD

— Criminal Minds (@CrimMinds_CBS) January 5, 2017

Delving deeper into the darkness isn't entirely fresh news for "Criminal Minds," but an upcoming episode that features guest star Tatum O'Neal will particularly seek that darkness. Actor Adam Rodriguez, who portrays Luke Alvez, said that the case in the upcoming episode is "super dark."

The mother of a serial killer, O'Neal's character will work with the BAU to thwart her son's planned murders. According to Rodriguez, this mother is attempting "to connect with her son and her son is just too far gone. This damage that was done, he blames her for it, she couldn't protect him from it," TV Insider reported.

Rodriguez went on to describe the upcoming episode as the "most gruesome" that he's seen in his tenure on "Criminal Minds." He assured viewers that watching the "tooth grinder" episode will not doubt make them cringe "with every part" of their body. This hints that the perpetrator is not just a serial killer, but a monster in every sense of the word.

"Criminal Minds" Season 12 will return for its tenth episode titled "Seek and Destroy" on Wednesday, Jan. 11 at 9:00-10:00 PM, ET/PT on CBS, SpoilerTV reported. In the episode, the BAU team investigates a string of home invasions in classy San Diego neighborhoods. As expected, the crimes will have several casualties. Watch the sneak peek below.

Any thoughts about the upcoming Reid-centric episode on "Criminal Minds?" How do you think it will affect him? Sound off in the comments section below!