Three sides of Mordor were bounded by mountain ranges, arranged in a rough rectangle: the Ered Lithui (translated as 'Ash Mountains') on the north, and the Ephel Dúath (literally, "Fence of Shadow") on the west and the south. In the northwest the pass of Cirith Gorgor led into the enclosed plain of Udûn. Sauron built the Black Gate of Mordor (the Morannon) across the pass, joining the Towers of the Teeth, two earlier guard towers built by Gondor to keep a watch on this entrance. The passage through the inner side of Udûn into the interior of Mordor was guarded by another gate, the Isenmouthe. Outside the Morannon lay the Dagorlad or Battle Plain.
In the interior within this mountainous border lay Sauron's main fortress Barad-dûr, the arid plateau of Gorgoroth, and Mount Doom. To the east lay the plain of Lithlad. A narrow pass led through the Ephel Dúath, guarded by Minas Morgul (earlier Minas Ithil). A higher, more difficult pass, Cirith Ungol, just to the north, was guarded by a tower originally built by Gondor. This pass, "the pass of the spider", was also blockaded by Torech Ungol, the lair of the giant spider Shelob. The fortress Durthang lay in the northern Ephel Dúath above Udûn.
Núrn, the southern part of Mordor, was less arid and more fertile. Streams here fed the salt Sea of Núrnen. Sauron's slaves farmed this region to support his armies.
Inside the Ephel Dúath ran a lower parallel ridge, the Morgai, separated from the Ephel Dúath by a narrow valley that Frodo and Sam followed northward after escaping from Cirith Ungol. Water trickled into this vale from the Ephel Dúath, and the text describes it as a "dying land not yet dead". The vegetation included "low scrubby trees", "coarse grey grass-tussocks", "withered mosses", "great writhing, tangled brambles", and thickets of briars with long, stabbing thorns. The fauna included maggots, midges, and flies marked with "a red eye-shaped blotch".
Adjacent to Mordor were, to the west, the narrow land of Ithilien (a province of Gondor); to the northwest the Dead Marshes; to the northeast Rhûn; to the southeast Khand; and to the south Harad.
In The Atlas of Middle-earth, Karen Wynn Fonstad assumed that the lands of Mordor, Khand, and Rhûn lay where the inland Sea of Helcar had been, and that the Sea of Rhûn and Sea of Núrnen were its remnants. This assumption stemmed from a First Age world-map drawn by Tolkien in the Ambarkanta, where the Inland Sea of Helcar occupied a large area of Middle-earth between the Ered Luin and Orocarni, with the western end being close to the head of the Great Gulf (later the Mouths of Anduin). The atlas was however published before The Peoples of Middle-earth, in which the Sea of Rhûn and Mordor exist already in the First Age.
Mordor was a relic of the devastating works of Morgoth, apparently formed by massive volcanic eruptions. It was given the name Mordor before Sauron settled there, because of its volcano and its eruptions. However, only Shelob had settled there before Sauron did.
Sauron settled in Mordor 1,000 years after the end of the First Age, and it remained the pivot of his evil contemplations for the whole of the Second and Third Ages of Middle-earth. In the northwestern corner of this land stood Mount Doom or Orodruin, where Sauron had forged the One Ring. Near Orodruin stood Sauron's stronghold Barad-dûr. After this time, Sauron was known as the Dark Lord of Mordor.
For 2500 years, Sauron ruled Mordor uninterrupted. Having wrought the Ring, it was from there that he launched the attack upon the Elves of Eregion. He was repelled by the Men of Númenor. Almost a thousand years later the Númenóreans under Ar-Pharazôn sailed to Middle-earth to challenge his claim to the title of "King of Men;" He realized that they would overthrow him if he decided to fight, so he let them capture him and bring him back to their island kingdom, where he eventually caused its destruction. (see Akallabêth). Immediately after Númenor's destruction, Sauron returned to Mordor as a spirit (the last of his living being having been bound to the One Ring) and resumed his rule.
Sauron's rule was interrupted yet again when his efforts to overthrow the surviving Men of Númenor and the Elves failed, and they formed a Last Alliance of Elves and Men whose army advanced on Sauron's land. A great battle took place on the Dagorlad in which Sauron's forces were destroyed and the Black Gate was stormed. The Barad-dûr was then besieged by the Alliance's forces. After seven years of siege, Sauron broke out and was defeated in a final battle on the slopes of Orodruin. After his defeat the Barad-dûr was levelled and great fortresses were built at the entrances to Mordor to prevent Sauron's return. For over a thousand years, Mordor was guarded by Gondor and remained desolate, although the watch was lessened somewhat during the reigns of some of the Kings.
Casualties from the Great Plague, during the reign of King Telemnar, were so high that the fortifications guarding Mordor were abandoned as the troops were called back to Gondor's cities. As the guard slackened, Mordor began to fill with evil things again. The Ringwraiths took advantage of Gondor's defeat in TA 1856 to re-enter Mordor and the final fortresses held by Gondor were abandoned and fell into ruin sometime after TA 1944. In 2002 Minas Ithil was conquered by the Nine Ringwraiths; and the fortifications that were supposed to defend Gondor from the menace inside Mordor were turned into a means of shielding Mordor. By the time Sauron returned into Mordor after his false defeat in Dol Guldur (in the events that took place at the time of Bilbo Baggins's quest), Mordor was protected too well to be captured by any military might that was available in Middle-earth at the end of the Third Age. In the north of Mordor during the War of the Ring were the great garrisons and forges of war, while surrounding the bitter inland Sea of Núrnen to the south lay the vast fields tended for the provision of the armies by hordes of slaves brought in from lands to the east and south.
During the War of the Ring, Sauron gathered all his forces to Mordor. After the Battle of the Pelennor Fields, a Host of the West went to the Black Gate. In the Battle of the Morannon Sauron sent his army to destroy the Men of Gondor and Rohan, but then Frodo Baggins destroyed the One Ring and Mordor fell. The Dark Tower, the Black Gate and the Towers of the Teeth were destroyed. Mount Doom exploded, clearing the sky over Mordor. Both Sauron and his Ringwraiths were apparently destroyed.
After the ultimate defeat of Sauron, Mordor became mostly empty again as the Orcs inside it fled or were killed. Crippled by thousands of years of abuse and neglect, but capable of sustaining life, the land of Núrn was given to Sauron's freed slaves. Gorgoroth remained desolate in the early part of the Fourth Age.
The armies of Mordor bore upon their armour and shields the red Eye of Sauron on a black field. This was also flown on the banner that accompanied the Lieutenant of the Tower of Barad-dûr when he confronted the emissaries of Gondor.
At the time of the War of the Ring, Sauron had gathered great armies to serve him. These included Easterlings and Haradrim, who spoke a variety of tongues, and Orcs and Trolls, who usually spoke a debased form of the Common Speech. Within Barad-dûr and among the captains of Mordor (the Ringwraiths and other high-ranking servants such as the Mouth of Sauron), the Black Speech was still used, the language devised by Sauron during the Dark Years of the Second Age. In addition to ordinary Orcs and Trolls, Sauron had bred a stronger strain of Orcs, the Uruk-hai, and very large Trolls known as Olog-hai who could endure the sun. The Olog-hai knew only the Black Speech.
Mordor had two meanings: "Black Land" in Sindarin, and "Land of Shadow" in Quenya. The root mor ("dark", "black") also appeared in Moria, which meant "Black Pit". Dor ("land") also appeared in Gondor ("stone-land"), Eriador, and Doriath ("fenced land"). The Quenya word for Shadow was "mordo".
A proposed etymology out of the context of Middle-earth is Old English morðor, which meant "mortal sin" or "murder". (The latter meaning was descended from the former.) It was not uncommon for names in Tolkien's fiction to have had relevant meanings in several languages, both languages invented by Tolkien, and actual historical languages. Tolkien was a professor of Anglo-Saxon, so his word roots tended to be Anglo-Saxon/Nordic/Germanic.
An art exhibition entitled “The Making of Mordor” at the Wolverhampton Art Gallery (2014) claims that the steelworks and blast furnaces of the West Midlands inspired Tolkien’s vision of, and his name Mordor. This industrialized area was known as “the Black Country”.
The name Mordor was translated as 魔多 (mó duō) in the traditional Chinese versions of the novel and films. The name translated aptly as "[a place where] demons are many". The simplified Chinese version of the film was translated as "魔都" (mó dū), which meant "the capital of the demons".
It was brought up in the third verse of Led Zeppelin's song "Ramble On" among other references to The Lord of the Rings.
In Chapter 10 of Outlaw Platoon (2012), his war memoir describing his service in Afghanistan, U.S. Army Ranger Sean Parnell observes: "Looks like we're at the gates of Mordor out here", which inspired the chapter's title "The Gates of Mordor".
In July 2015 NASA published photographs taken as the New Horizons space probe passed within 7000 miles of Pluto. A photo of Pluto's largest moon, Charon, shows a large dark area near its north pole. The dark area has been unofficially called "Mordor".