Which state was admitted to the Union as a slave state as part of the 1820 compromise that also admitted Maine as a free state?

Missouri

The Missouri Compromise was a pivotal legislative agreement designed to maintain a delicate balance of power between Northern and Southern states in the U.S. Senate. While Missouri entered as a slave state, the compromise also established the 36°30′ parallel as the boundary line, prohibiting slavery in any future territories north of that latitude. This "line in the sand" helped delay the sectional conflict over slavery for several decades.