Which Tolkien novel features the fictional land of Mordor?

J.R.R. Tolkien’s The Lord of the Rings features the fictional land of Mordor.

Mordor is the realm where Sauron establishes his stronghold during the War of the Ring. It lies in the southeast of Middle-earth and is associated with volcanic terrain, ash, fortified passes, and Mount Doom. The One Ring must be taken there because only the fires of Mount Doom can destroy it.

The Lord of the Rings follows the Ring-bearer Frodo Baggins and his companions from the Shire through a widening conflict involving Sauron, the Free Peoples, and the fate of Middle-earth. Mordor becomes the destination of the central quest, although the narrative also follows major events elsewhere, including the defense of Gondor and the battle at Helm’s Deep.

Mordor is mentioned in Tolkien’s broader mythology, but The Lord of the Rings gives it its most extensive narrative role. The name should not be confused with Isengard, another major fortress associated with the War of the Ring.