WARNING: The following contains spoilers for WandaVision, streaming now on Disney+.
A recent high-res movie poster for the upcoming Dr. Strange in the Multiverse of Madness is causing quite a stir since it may reveal that the true villain of the film is not Wanda, but actually the elder god Chthon. A close-up of one of the shards of glass within the poster seems to show the reflection of a demonic looking creature that bears a striking resemblance to the villain.
Chthon is the author of the Darkhold which was just introduced in WandaVision last year, so it is seems possible the Elder God is spurring Wanda's evil spree from behind the scenes and slowly possessing her like he did in the comics. To add fuel to the rumor fire, a newer TV spot seems to support the theory and shows a scene of America within a demonic-looking temple that has runes and symbols similar to those found within the Darkhold. WandaVision might have potentially been foreshadowing this powerful villain's film MCU debut that whole time.
Agatha Harkness reveals an extensive amount of information to Wanda about her powers and the Darkhold toward the end of WandaVision. During this, she flips through the book to the page that describes the Scarlet Witch and the same imagery from that page can be seen on the wall of the temple within the Multiverse of Madness tv spot. The architecture of this temple is also eerily similar to the design of Agatha's basement lair with ram statues appearing along the walls with the ram being a common symbol of Chthon from the comics.
She also tells Wanda that the Scarlet Witch should be a myth and her chaos magic was gifted to her. Agatha does not say who gave this "gift," but this could be seen as a clear reference to Chthon as in Marvel Comics' lore he is the one who imbued Wanda with chaos magic. Wanda is also the last person to be in possession of the Darkhold at the end of WandaVision, so it would make sense that Chthon would try to use this as his opportunity to possess her.
In the comics, Chthon marked Wanda as his potential vessel when she was born within a lab on Wundagore mountain. This mountain is a key detail as well since there have been several scenes of Dr. Strange on top of a snowy mountain in many of the Multiverse of Madness trailers and TV spots. Many fans are now seeing this as potentially being Wundagore and that Strange is going there to confront the Elder God. This mountain is where Chthon's temple, the Darque Hold, is located within the comics and it wouldn't be surprising to find out the temple that America is in during the film is, in fact, the Darque Hold.
Chthon later does attempt to use Wanda as his vessel on Earth during Avengers Vol. 1, but is eventually defeated by the Avengers. The Darkhold and Wanda's embrace of chaos magic during WandaVision is more than likely Marvel Studios' way of translating Chthon, and his history with Wanda, to the Marvel Cinematic Universe. Wanda is desperate to find her children and that fact definitely makes her an easy target for Chthon's influence. There is even speculation that Wanda's children aren't the ones calling out to her from the Darkhold, but rather Chthon using a spell to mimic their voices.
This kind of possession would definitely explain the drastically more evil Wanda that has been depicted within the various trailers and tv spots for Multiverse of Madness. The bloody-faced Wanda shown during the Multiverse of Madness trailers is in no way the same Wanda Maximoff that MCU fans have come to know and love over the years. While her apparent villainous turn could be spurred by a strong desire to have her children, it would make more sense to introduce Chthon as the true master of evil behind the scenes.
Wanda did some tragic things during the events of WandaVision, but not because she is outright evil. Wanda was grieving during the series, and in many ways, she still is going into the events of Multiverse of Madness. Being in a vulnerable and emotional state while also being alone with the Darkhold is the perfect opportunity for Chthon to make his big move. It is also the perfect opportunity for the MCU to come full circle and fully explain the origins of Wanda's powers within the MCU.
Looking back on WandaVision it is clear there are some heavy hints about the origins of Wanda's chaos magic and the future the Darkhold has within the MCU. While there are still only tiny slivers and rumors to reflect on, for now, the full picture will be clear when Dr. Strange in the Multiverse of Madness debuts in May.