Khazad-Dûm
Khazad-Dûm, also known as Moria, is the great abandoned mansion of Durin's Folk. Once the grandest and most famous Dwarven city in Middle Earth, it was ruined and abandoned after the awakening of Durin's Bane in T.A.1980. Afterwards, the city was ransacked and colonised by roaming bands of Orcs from the Misty Mountains, who remain there to this day.
Summary
After Durin the Deathless awoke under Mount Gundabad, he came upon a shimmering lake in the vale of Azanulbizar. There, he saw a crown of stars reflected in its waters, and he began to excavate the caves in the mountains above. This was the foundation of Khazad-Dûm. The Dwarves of Durin's Folk prospered for millennia in their great mansions, their numbers swelled at the close of the First Age by refugees from the Blue Mountains. The Dwarves of Durin's Folk enjoyed close relations with the Elves of Eregion, but after the devastation of that land following the War of the Elves and Sauron, the Dwarves shut their gates and became increasingly secluded.
In their isolation, the Dwarves of Khazad-Dûm slowly dwindled. Their holdings retreated as bands of Orcs overran outlying settlements, and the economy of Khazad-Dûm became increasingly fixated on the mining and refining of mithril. The Dwarves delved ever deeper in search of the precious metal, until in T.A. 1980 they awoke a nameless terror deep beneath Caradhras. Durin VI was slain, as was his son Náin I. The Dwarves fled Khazad-Dûm, leaving behind a looming, empty ruin which soon became known as Moria, the 'Black Pit'.
In time, bands of Orcs from the north entered Moria, plundering its riches and colonising its empty halls. Little now remains of the great wealth of Khazad-Dûm, much of it having been given as tribute to Mordor. In T.A. 2790 the exiled King Thror entered Moria, but was slain by the Orc chieftain Azog. His death precipitated the War of the Dwarves and Orcs, a six year conflict which culminated the Battle of Azanulbizar outside the gates of Moria. The Dwarves claimed victory, and the Orcs of the Mountains were greatly reduced, but the Dwarves were wary of Durin's Bane, and did not re-enter Moria. In T.A. 2989 an expedition set out from Erebor to reclaim Moria, led by Balin. They were able to reclaim several lost artefacts and set up a small colony in the 21st Hall. Balin's colony was overrun by Orcs in T.A. 2994, and no Dwarves have returned since.