Khazad-dûm is the Dwarven underground city called Moria in J.R.R. Tolkien’s writings.
Khazad-dûm was the greatest mansion and realm of the Dwarves in the Misty Mountains. In the Common Speech of Middle-earth, its later name is Moria, meaning the Black Chasm or Black Pit in Tolkien’s constructed linguistic system. The mines were famed for wealth, craftsmanship, and mithril.
The Dwarves abandoned Moria after awakening a Balrog deep beneath the mountains. By the time the Fellowship passes through it, the city is deserted and dangerous. The tomb of Balin and the attack by Orcs make the underground journey one of the darkest sections of The Fellowship of the Ring.
Erebor is the Lonely Mountain, not Moria. Gondolin and Nogrod are important locations from earlier ages of Tolkien’s mythology, but neither is another name for Khazad-dûm.