Which Sith Lord is most associated with the main villain role revealed in Episode IX?

Episode IX reveals the resurrected Emperor Palpatine as the hidden mastermind behind the First Order, the creation of Snoke, and the Final Order fleet on Exegol. By contrast, Darth Vader dies redeemed in Episode VI, Darth Maul’s arc is confined to the prequel era and animated stories, and Darth Plagueis is only referenced in Episode III and never appears in the sequel trilogy. Palpatine’s return—supported by the Sith Eternal cult—links the entire Skywalker saga, from his rise in The Phantom Menace to his bid to reclaim ultimate power in The Rise of Skywalker. Ian McDiarmid reprises the role, and the plot centers on his attempt to exploit the Force dyad between Rey and Kylo Ren and to pass his dark legacy to his unsuspecting granddaughter.