Khazad-dûm is the Khuzdul name for Moria in J.R.R. Tolkien’s works.
The Dwarves founded Khazad-dûm beneath the Misty Mountains, and it became the greatest of their mansions. The city’s wealth and power were closely tied to mithril, a rare and valuable metal found in its mines. The Dwarves’ excavation eventually awakened a Balrog, after which they abandoned the realm.
By the time of The Lord of the Rings, the name Moria is associated with the abandoned and dangerous underground complex. The Fellowship enters it while attempting to cross the mountains after the passage over Caradhras becomes impossible.
Khazad-dûm is often confused with Erebor, the Lonely Mountain, which is another famous Dwarven kingdom. Gondolin and Angband are also underground or fortified places in Tolkien’s wider mythology, but they belong mainly to the histories of the First Age.