The fictional land ruled by Sauron in J.R.R. Tolkien’s The Lord of the Rings is Mordor.
Mordor lies in the southeast of Middle-earth and serves as Sauron’s principal realm during the War of the Ring. Tolkien describes it as a harsh region surrounded by mountains, with volcanic Mount Doom at its heart. The geography helps explain why the land is both strategically valuable and difficult for enemies to enter.
Mount Doom is especially important because the One Ring was forged there and can be destroyed only in the fires where it was made. Frodo Baggins and Samwise Gamgee travel into Mordor after crossing the hostile terrain of the Emyn Muil, the Dead Marshes, and Cirith Ungol.
Mordor is not the same as Gondor or Rohan. Gondor is a major human kingdom, Rohan is the horse-lords’ realm, and both oppose Sauron during the conflict described in the trilogy.