Even though we still don’t yet know exactly what happened between Rio and Agatha, we know that Rio considers it to be her biggest scar. She says so herself in the previous episode and makes it clear that it was something she didn’t want to do. Each trial seems to center around a witch and their biggest trauma. The first was Jen (Sasheer Zamata) losing her powers. The second was Alice (Ali Ahn) and her generational curse. If Rio is Death, there’s a chance that she had to take Agatha’s son from her, which would definitely constitute a trauma worthy of the Witches’ Road.
There’s also the fact that Evanora’s ghost threatens to take Agatha. Rio’s reaction toward Agatha’s mom indicates that there’s some sort of past there, Rio hates her, and seems to even be slightly afraid of what she might do to Agatha. Evanora says that she wishes she killed Agatha when she had the chance, so maybe Rio had to stop her at some point. We still don’t yet know how Agatha and Rio met, but flashback images circulating the internet seem to indicate that it was well into the past.
After the trial ends, we don’t see Rio. It’s likely she stays behind to tend to Alice and her spirit as Death would, but she could also be processing what she just saw and the fact that Agatha’s mother goaded her into siphoning power once again. The Road also turns an odd shade of green after the trial is done – for the Green witch herself, perhaps?
Another theory is kind of a two-parter. Part of this theory argues that this trial was actually Teen’s. The setting for the trial is that of an ‘80s horror movie, a genre whose characters are typically teens. The coven is all dressed to match, with Alice even joking that playing “kiss, marry, kill” is the way to end the trial.
Then there’s the fact that the Teen loses his spellbook. And how quickly the coven turns on each other. Teen is visibly upset by this, and we see his powers fully manifest in a moment of sadness and anger after seemingly losing this family he had made for himself. Because of the Sigil, it’s not likely the Road could formulate his trial around losing Wanda. But it can show him what it’s like to lose his other family, forcing him to shake the crutch of the spellbook, and possibly even break the Sigil, finally embracing his power as his own.
The other part of this theory is that Teen knows more than he’s letting on, and he brought all of these witches here for his own plan. Teen has been the one to kickstart the trial each time. He’s the one who read the card that summoned the poisoned wine. He played the cursed record. He found the Ouija board and read its instructions. Now that we know for sure that he’s Billy, he’s likely after a way to bring back his mom.
