https://www.cnet.com/news/star-wars-episode-9-rise-of-skywalker-sees-emperor-palpatine-rise-again/

The Episode 9 trailer came out at Star Wars Celebration in Chicago, and when the lights came back on the audience was stunned to see actor Ian McDiarmid standing on stage. McDiarmid played the evil Palpatine in the original trilogy and the prequels, but hasn't been seen since -- Palpatine was long dead by the time periods portrayed in The Force Awakens, The Last Jedi and The Rise of Skywalker.

In the prequels, Palpatine was the ambitious galactic senator who provoked a war and perverted the Galactic Republic. In the original trilogy he had become emperor, ruling the galaxy with an iron fist. We first saw Emperor Palpatine in The Empire Strikes Back, and in Return of the Jedi he was cast to his death by a repentant Darth Vader. Before his apparent demise, he gloated to Luke Skywalker that "only now, at the end, do you understand..."