I think the guy with the horror mask is not the Sith master but the Sith apprentice, and Amandla's character (or one of her characters 🤔) is the person they're recruiting as the apprentice prepares to make a move on their master. With how the whole Rule of Two works, I think it would make sense that the apprentice is already plotting behind the scenes for their moment of triumph, having someone backing them for when they do ultimately make their move against the master.
Perhaps envisioning a 2-1 situation: the acolyte and the apprentice vs. the master. As for whether that character is Plagueis, Tenebrous, or someone new, I don't know. However, I think Plagueis and/or Tenebrous absolutely have to be mentioned in some regard at some point or another.
There are rumors as well that Amandla is playing two characters. Whether or not it's the same person but with two personalities, or two twins, I don't know. Considering the TV spot where she's seen telling the Jedi, "Hey, it wasn't me!" which I think is connecting to the idea of Amandla's character doing these Jedi murders. If it is twins, what if the twins were separated and one of the twins (the bad one) knows the other exists, and is doing these murders knowing that the other twin will be framed for it?
I'm big on the theory that the Jedi come to this village and do take a child, a move that upsets that coven of witches we've seen talked about in trailers and discourse. Perhaps the child they took was the one who went on to become a Jedi (but then left the Order later on in their life for whatever reason), and the one who stayed was the one who became dark.
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