As much as I want to vote for Yoda, I gotta go with Darth Plagueis.
The Acolyte is a mystery-thriller that will take viewers into a galaxy of shadowy secrets and emerging dark-side powers in the final days of the High Republic era. ... but the forces they confront are more sinister than they ever anticipated.
Granted, I'm sure it could be something new and different. Or at least something covered within the High Republic comics (maybe) that have yet to be fully explored. But the fact that we're within 100 years of the Skywalker Saga, I'm leaning towards seeing the Sith emerge somehow.