Largely, the history of the Mror Holds is covered in the article on Dwarves.
Following the rise of Galifar, the dwarves were conquered by Prince Karrn, and they were so mired in their feuds that they were unable to present a united front to push the humans back. Galifar had little actual interest in the Ironroot Mountains themselves, however, and so the dwarves were left as largely independent clans that paid taxes to the crown and were subject to it's laws.
Left to their own devices during the Last War, the dwarves expanded and eventually discovered ancient dwarven ruins beneath the mountains, entire subterranean cities, which they named Sol Udar, the Realm Below. During this time, Karrnath was imposing heavy taxes on the clans but otherwise not interacting with them, and so riding the high of their discoveries they convened the Iron Council and declared their independence. Karrnath attempted to retaliate, but they didn't really have the resources to seriously press the dwarves, and so the Mror Holds were born.
For the last few decades of the Last War, the Mror were fighting their own war: Dol Udar, the War Below. In typical tropish fashion, the dwarves dug too deep and eventually discovered why the dwarves of the Realm Below had never attempted to contact them; they had been annihilated by the forces of the Daelkyr, Dyrrn, the Corruptor.
Life & Society
The Mror are a feudal society, split into holds, spires, and families. There are twelve holds in the Ironroot Mountains, each governed by one of the twelve ruling clans from which they derive their names. These holds are then split into smaller territories called spires, each of which is ruled by a smaller clan that is tied to the ruling clan of the hold by kinship and marriage. Within these smaller territories, land is held by a clan or individual family, and most of the businesses are family businesses.
The dwarves of the Mror holds generally engage with their history through stories, with the clans and families taking the roles of characters within these stories. Mror stories generally refer to characters solely by their clan or family name, so in a story about Mroranon, the main character is literally named Mroranon. To the Mror, it doesn't matter which Mroranon dwarf it was that did the deeds portrayed in the story, and it doesn't even really matter where or when it took place. The stories that the clans keep help them to model the behaviours and values that they want to represent, and strive to live up to the examples set in their clans tales. To the Mror, family is an extension of your own identity, and you put your family first. This merging of individualy identity with the family is also the cause of much of the feuding that happens in the Holds, if you wrong a single dwarf, you've wronged their entire family, and your entire family takes the blame for the slight.
Much like how the Aereni and Tairnadal have their own ways to deal with their long lives, the Mror dwarves' attitudes towards family is how they deal with theirs. Most dwarves can live up to about 350 years of age if nothing kills them, not as long as an elf, but far longer than a human. The vagueness of their stories is a way of coping with this, they don't need to meticulously record everything that happens, instead they focus only on remembering the best moments that life has to offer. Individuals come and go, but the family endures.
The Mror can be seen as boastful by other peoples in Khorvaire. They love stories, deeply, and much like the Heirs of Dhakaan, they prefer stories about real events over pure fiction. Unlike the dar, however, they aren't quite as concerned about everything being factual, stories should be entertaining and as long as the events depicted are based on true events, the actual details aren't important. Their love of stories is important to them, and they expect others to share their own stories as well, often boasting on behalf of their companions if they won't do it themselves. After all, our successes should be celebrated!
Government and Politics
The Mror Holds is ruled by the Iron Council, which has representatives from each of the 12 ruling clans. House Kundarak has a seat on the council, but is officially only an observer and has no actual say in anything that the council decides. The ruling clans form the foundation of society in the Mror Holds, every citizen owes allegiance to one of them, whether they are directly part of one of the clans, or part of a smaller clan that pays fealty to them.
The Iron Council is based in Krona Peak, which makes it the capital of the Mror Holds. The loss of Clan Noldrun was devastating to the Iron Council, as they no longer have an odd number of clans to prevent ties. Clans Toldorath and Tordannon have suggested giving the Jhorash'tar orcs a seat on the council, but the other clans largely oppose this.
Religion
The dominant religion of the Mror Holds is the Sovereign Host. The dwarves believe that they are the chosen people of the Sovereigns, in particular Kol Korran and Onatar. The dwarven words for "commerce" and "war" are "Kol" and "Dol" respectively, and it's not a coincidence that the names of the Sovereigns incorporate these words. The Mror believe that when the founding clans were exiled from Sol Udar, the Sovereigns names were stolen from them by The Traveler, and that the sovereigns went to the dwarven clans and asked that they remember their names.
Clan Kolkarun believes that their founder was literally Kol Korran, whilst Clan Mroran (who have strong ties to Clan Kolkarun) believe that Kol Korran came to them first to ask that they remember his name, in exchange for the gift of commerce. Clan Doldarun believes that their founder was the child of Dol Arrah and Dol Dorn. The dwarves of the Mror Holds had shrines to the Sovereign Host long before the humans came to the mountains, but like with many aspects of dwarven history, nobody knows exactly when they adopted the faith of ancient Pyrine. After all, the details don't matter.