Episode eight of WandaVision, titled "Previously On," takes us all the way back to the beginning; not just of the show, but to Wanda's first interaction with the Stark Industries bomb that would change her life. Up until now, we've been under the assumption that Wanda's powers, at least in the Marvel Cinematic Universe, are a direct result of the HYDRA experiments she took part in along with her brother prior to Avengers: Age of Ultron. Through the memories that Agatha Harkness makes Wanda relive during the episode, however, we realize not everything has been as it seems.
Agatha wants to know what makes Wanda tick and how Westview came to be, so she makes the latter relive some of the most traumatic moments in her life to get answers. First up is the death of her parents, courtesy of a mortar shell through their apartment during TV time. It's when the second Stark Industries bomb arrives that Agatha poses the question of, "Did you stop that bomb?," to which Wanda doesn't have an answer, assuming it didn't go off because it was defective. "You used a probability hex," Agatha says. "What I see here is a baby witch obsessed with sitcoms and years of therapy ahead of her," she continues after Wanda responds they were safe for two days.
The duo then continue through Wanda's memories, stopping off at the HYDRA experiment. Prior to WandaVision, all we knew about the experiments on the Sokovians was that Wanda and Pietro were the only two subjects to survive direct contact with the Chitauri scepter, which was later revealed to be powered by the mind stone. In the memory, not only does the mind stone reveal its true form to Wanda, but she also sees a vision of her Scarlet Witch persona as she's engulfed in the warm yellow light. Neither of the HYDRA doctors can explain what happens to her, and the events aren't captured on their security cameras. "So, little orphan Wanda got up close and personal with an Infinity Stone that amplified what otherwise would've died on the vine," Agatha concludes, although she looks even more confused than before. "I have a theory, but I need more."
We don't learn of Agatha's theory until after she discovers how Wanda created Westview out of nothing. "You have no idea how dangerous you are. You're supposed to be a myth, a being capable of spontaneous creation. And here you are, using it to make breakfast for dinner," Agatha calls out as she holds Wanda's children hostage. "Your children, Vision, this whole little life you've made. This is chaos magic, Wanda. That makes you the Scarlet Witch."
This is the first time Wanda has ever been referred to as the Scarlet Witch in the MCU. With the idea that Wanda has been a witch all along and now controls chaos magic, versus a mutant as her comic-book origins state or strictly from the mind stone as we previously believed, a future appearance by Chthon seems like the next logical step. The elder-god has comic-book ties to the Darkhold, which is the book we see in Agatha's basement in episode seven. He is also the one who amplifies Wanda's powers, similar to the mind stone on WandaVision. Since Agatha is hundreds of years old and speaks of the Scarlet Witch as folklore, it seems safe to say Wanda might not be the first incarnation of the being, which is where Chthon could also come into play. Regardless of what happens in the finale, Marvel retconning Wanda's origins is an interesting route, although we can't blame them since they couldn't use her initial comic-book origins (Marvel Comics retconned those in 2014) thanks to their original deal with Fox.