I'm familiar with Darth Raven from the KOTOR game and the Raven book from the Legends, but he was alive way earlier, no? Like, thousands of years before the events of the Acolyte, I think... If we're going down that path, canonization of Darth Tenebrous would make a lot of sense to me, and I would love to see that happen! In the legends, he was the master of Palpatine's master, and fits well into the timeline, too.
Sol is definitely hiding something, but in my opinion, it has to do with what the Jedi really did on that witches planet. It's pretty clear that the previous flashback episode was shown from Osha's point of view and her memories of the events (so potentially a version of events the Jedi wanted her to believe in), so we weren't shown what actually happened there. I am pretty sure the Jedi were responsible for the deaths of the witches and the destruction of the temple. One of the Jedis willingly drinking the poison and apologizing to Mae gives some further support to that theory, too. Something dark happened there, and the Jedi were directly involved with it. So my current guess is that Osha will found out what really happened there, find out it's all Jedi fault, turn to the dark side fully, and kill her former master Sol, becoming the Acolyte.
By the way, there were Sith-like hooded figures standing in the room during the ritual of the witches in that flashback episode, and I think there were also some Sith-looking markings on the walls of that witch temple, too. I wouldn't be surprised if there's a connection there, and maybe the witches even used some Sith rituals to create the twins!