Red Eye Mountain, formerly known as Karak Ungor, or the Delving Hold is a massive fortress-city of the Night Goblins of the Red Eye Tribe, which was conquered during the onset of the Goblin Wars. At the Dwarfs' height during the Golden Age, Karak Ungor boasted one of the deepest and richest mines in all of Karaz Ankor. As the ore and mineral deposits were finally dug out completely, the Dwarfs simply abandoned the tunnels, never expecting the consequence of such ill-vigilance would have upon their kingdom. During this time, the War of Vengeance was nearing its end and the Elves were a spent force, the Greenskins still kept to the Dark Lands, and the Skaven were still unknown. The occasional Troll wandering through the mines was seen as the only threat to the dominion of the Dwarfs.
Soon after the final victory over the Elves during last few days of the war, the Worlds Edge Mountains suddenly shook with massive earthquakes and volcanic eruptions. Fissures erupted within many of Karak Ungor's underground tunnels, through which surged the Night Goblins of the Red Eye Tribe. King Morek Stonehammer and the army of Karak Ungor were still in the west when this all happened, and could not do anything to stop the Night Goblins from conquering all in their path. They returned to find that Karak Ungor had fallen, and though they made fifty attempts to breach the gates, they were betrayed by the quality of their own works and were easily repulsed. In despair they retreated to Karak Kadrin and Zhufbar, hoping one day to recover their fallen hold from the clutches of their hated enemy.
Karak Ungor was later named Red Eye Mountain, after its new tenants, and over 500 years passed before the Dwarfs made another attempt to reclaim it. King Skorri Morgrimson led the assault known as the Battle of a Thousand Woes, where the Dwarf succeeded in capturing the southern valley and gate, but were unable to enter the citadel itself. After substantial losses, Skorri's army was forced to withdraw to Karaz-a-Karak in failure. Skorri passed the crown to his younger cousin Rogni Stonehammer, and took the Slayer's Oath and died shortly afterwards.
Ulther Stonehammer is the current Prince of Karak Ungor, having recently succeeded his father King Ulfar as head of the Durazgrund Clan. He has sworn never to take the title of King until Karak Ungor is reclaimed. Prince Ulther has since founded an elite force of Dwarf warriors known as the Dragon Company. Normally based at Karaz-a-Karak, the Dragon Company have sworn an oath to retake Karak Ungor, and to this end they roam the northern Worlds Edge Mountains raiding Greenskin settlements to achieve that goal.
The Black Orc Warboss Grimgor Ironhide once captured the stronghold while looking for a holiday spot preferably with lots of Dwarfs, Other Greenskins and Skaven to kill, and thankfully the Karak was reasonably close to Karak Kadrin the Slayer Hold or in Grimgor's eyes Suicidal Lunch and Hellpit the Grand Under-Empire seat of Clan Moulder was a bit of a way off but Grimgor didn't mind. When taking the Hold from the Red Eye Grimgor wasn't interested in a long siege so he raided the area and captured the supreme Chieftain of the area and ripped and ate his head off in front of the city's defenders. Saying that the terrified Night Goblins opened the gates quickly after that incident would be an understatement. As far as the lore goes the Tribe was allowed to live after this incident.
Source
- Warhammer Armies: Dwarfs (8th Edition) -- p. 15 - 20