The underground realm called Khazad-dûm in J.R.R. Tolkien’s works is Moria.
Khazad-dûm was the greatest of the Dwarven mansions and was located beneath the Misty Mountains. Elves called it Moria, a name that later became associated with its abandoned and dangerous state. The Dwarves mined mithril there, making the realm exceptionally wealthy, but their mining awakened a Balrog known as Durin’s Bane.
By the time the Fellowship enters Moria in The Lord of the Rings, the Dwarven kingdom has fallen. The Fellowship travels through its ruined halls and encounters Orcs, a troll, and the Balrog. Gandalf confronts Durin’s Bane on the Bridge of Khazad-dûm, leading to his apparent death and later transformation. Moria is distinct from Erebor, the Lonely Mountain, which is another famous Dwarven stronghold in the north-east.