WARNING: The following contains spoilers for WandaVision Episode 6, "All-New Halloween Spooktacular!" now streaming on Disney+.

Nothing happens by accident on WandaVision, whether it's the show Disney+ viewers are watching or the one S.W.O.R.D. is monitoring. So, when the "cast" is suddenly clad in Halloween costumes, there's as much hidden meaning to be found in Westview as there is candy. And one mysterious background character in the series' latest installment, "All-New Halloween Spooktacular!," may turn out to be either a trick or a treat for theorizing fans.

In the episode, the audience sees just how many people are stuck in Wanda's bubble and under her control. As it turns out, Westview is a bigger and more heavily populated place than both Vision and viewers might've previously thought. For the first time, Pietro notices that kids besides Wanda's twins are part of the action. Meanwhile, the townspeople in Wanda's proximity get more fully imagined lives so they can interact with her realistically -- well, realistically for whatever era of sitcom Wanda's copying -- but those on the outskirts barely warrant a thought and have become little more than human props. We also know from Vision that Wanda has, either purposefully or subconsciously, chosen everyone's alter ego for them.

RELATED: WandaVision Hilariously Reveals What Halloween Was Like in Sokovia

The main characters' costumes are obvious allusions to their comic-book counterparts. Wanda, Vision, Pietro, Billy and Tommy are all wearing early iterations of their 2D superhero uniforms. Elsewhere around town, neighbor Herb is Frankenstein's monster, Agnes is not so subtly done up as a witch and various trick-or-treaters are styled as a mummy, a pirate, a hippie, a wizard and Cleopatra. There's also a butterfly, a bag of popcorn, an angel and at least two skeletons, costumes that might be symbolic of something. Still, all of these characters mingle as extras on a TV show should, except for a hooded and masked figure who looms over Wanda's shoulder and doesn't move for the entirety of one scene.

The moment is too oddly specific to be nothing more than random background business. Exactly who the creepy loiterer is and whether Wanda's in command of him are open questions. The series could be having a bit of fun of with fans who are prone to scrutinizing every detail, but for a show in which every detail is so loaded with story potential and comic-book reference, that would seem almost cruel. The figure, who at least bears a passing resemblance to death, could also signal someone's approaching demise, or even the coming destruction of Wanda's world. Or it could be the first appearance of an oft-rumored and much anticipated new villain: Mephisto.

RELATED: WandaVision Explains Scarlet Witch's Comics Costume

At this point, even WandaVision viewers who haven't read the comics have heard the name "Mephisto" and have probably learned along the way that he's an extra-dimensional demon whose story intertwines with Wanda's. The show has been dropping Mephisto-related hints since it began, and there's a case to be made that he could be just about any one of the peripheral characters. Early guesses held that he was biding his time as Agnes' unseen husband, Ralph. Others surmised that, since Mephisto can shapeshift, he might be hiding in plain sight as one or all of the many animals that have appeared in Westview. Audiences have also suspected neighborhood queen bee, Dottie. Then, the evidence began to mount that Pietro might actually be Mephisto (or some other Marvel villain) in disguise.

Wanda and Pietro in their Halloween costumes

WandaVision's sixth episode goes a long way to confirm but also confuse most of fandom's Agnes-is-Agatha Harkness and Mephisto-is-(fill in the blank) theories. Agnes' maniacal laugh is certainly suspicious, and it's highly likely she is actually Agatha. But she also tells Vision that "all is lost," and even if she's involved in the greater scheme of things, we still can't be sure she's evil.

Pietro and the hooded Halloweener can be seen in the same frame, but we don't see Dottie and still haven't met Ralph. The biggest clue perhaps of the whole series so far is Pietro's aside to the twins. "Unleash hell, demon spawn!" he yells as they run off to claim their candy. Canonically, Wanda creates Billy and Tommy by stealing some of Mephisto's lifeforce, meaning the kids are quite literally demon spawn. The line at least strongly suggests WandaVision is borrowing from the comic-books.

KEEP READING: WandaVision's Hexagons May Be Further Proof of Mephisto

Pietro's shady behavior, including not remembering his and Wanda's shared past and wanting to know how she's deploying her magic, means he could be a nefarious imposter. However, it could also be explained away if he's just another re-animated corpse. Vision can't remember his previous life, either, and he's equally concerned with how and why Wanda's doing what she's doing.

Whoever the cloaked figure in the background of the episode is, he was watching the Sokovian twins' conversation with great (and menacing) interest. Maybe Wanda's not the only powerful being capable of creating alternate realities and raising the dead? Mephisto could do it, but so could Nightmare, a dark lord from Marvel Comics who rules the dream world. The word "nightmare" is uttered twice in the episode and the Town Square Scare definitely turns into one. Despite all the cameos and Easter eggs that have kept fans theorizing on overdrive, with only three more episodes left, WandaVision can't shroud its ultimate villain in symbols and secrecy much longer.

Written by Jac Schaeffer and directed by Matt Shakman, WandaVision stars Elizabeth Olsen as Wanda Maximoff/Scarlet Witch, Paul Bettany as Vision, Randall Park as Agent Jimmy Woo, Kat Dennings as Darcy Lewis, Teyonah Parris as Monica Rambeau and Kathryn Hahn as Agnes. New episodes air Fridays on Disney+.

KEEP READING: A WandaVision Guide: News, Easter Eggs, Reviews, Recaps, Theories and Rumors