Star Wars set up Darth Plagueis' master years ago, and he could finally appear on-screen next year in The Acolyte. The master of the master who trained Emperor Palpatine, Darth Tenebrous was an influential Sith Lord descended from the long line of masters and apprentices dating back to Darth Bane, who instituted the Rule of Two. Now, The Acolyte has an opportunity to flesh out this early Sith Lord even further in the established canon.
https://screenrant.com/star-wars-darth-tenebrous-setup-acolyte/