The Dark Iron Clan was ruled by the Sorcerer-thane Thaurissan and was unsuccessful in wresting control of Ironforge from the Bronzebeard Clan and the Wildhammer Clan. Now ruled by Emperor Dagran Thaurissan, the Dark Irons live in Blackrock Mountain as the slaves of Ragnaros. Dark Iron dwarves are neither mountain dwarves nor hill dwarves but rather considered just shadowy, wandering sorcerers. ( )
Like all dwarves, the dwarves of the Dark Iron Clan are descendants of the earthen.
Just prior to the War of the Three Hammers the Dark Iron Clan, ruled by the Sorcerer-thane Thaurissan, inhabited the darkest, deepest crevices of Ironforge. The Dark Iron Clan was unsuccessful in wresting control of Ironforge from the Bronzebeard Clan and Wildhammer Clan during the War of the Three Hammers.
Thaurissan and his Dark Irons vowed revenge against Ironforge. He took his followers south and founded a city (which he named after himself) within the beautiful Redridge Mountains. Despite years of prosperity, Thaurissan and his sorceress wife, Modgud did not soften in their desire to defeat the Bronzebeards and Wildhammers; they launched a two-pronged assault against both Ironforge and Grim Batol. Thaurissan was routed by Bronzebeard at Ironforge, and retreated back to the Redridge Mountains. Modgud's assault on Grim Batol was marginally more effective, though she herself fell in battle.
As the combined forces of Ironforge and Grim Batol marched on Thaurissan, he attempted to summon an ancient power sleeping beneath the Redridge Mountains. Ragnaros the Firelord, immortal lord of all fire elementals, had been banished by the Titans when the world was young. Now, freed by Thaurissan's call, Ragnaros erupted into being once again. Ragnaros' apocalyptic rebirth into Azeroth shattered the Redridge Mountains and created a raging volcano at the center of the devastation. The volcano, known as Blackrock Mountain, was bordered by the Searing Gorge to the north and the Burning Steppes to the south. Though Thaurissan was killed by the forces he had unleashed, his surviving brethren were ultimately enslaved by Ragnaros and his elementals. They remain within the Spire to this day.
(above paraphrased, ©2005 Blizzard Entertainment)
Modern Dark Irons
The modern Dark Iron clan serves Ragnaros. They are centered in the depths of Blackrock Mountain, in their dark capital of Shadowforge City. A great deal of Dark Iron activity can be seen in the Searing Gorge and in the Badlands. There have also been sightings in the Wetlands, Loch Modan, the Burning Steppes, and Dun Morogh.
Dark Irons are enemies of both the Alliance and the Horde. They seem to have helped Sicco Thermaplugg in the invasion of the Troggs in Gnomeregan, as some Dark Iron Dwarves and even an Dark Iron Ambassador can be found in the poisoned gnome capital. It's unclear what exactly they did, though.
Some Dark Irons have broken ties with the clan in order to seek out business relations with the Alliance and Horde. This group is known as the Thorium Brotherhood. The Dark Irons have proclaimed the Brotherhood an enemy of the Clan for their relations, and the two groups are now at war.
The Dark Irons have recently heard of the Forest Troll Serpent Goddess Ula-tek being reborn in Zul'Aman, and reckoned that this (deemed) Old God might get along well with their Elemental Lord Ragnaros. They have since sent emissaries to Ghostlands in order to get to know each other and become allies.
Dark Iron dwarves easily stand out from the other two clans. They physically resemble Bronzebeard dwarves, but they are less stocky and more dexterous. Their skin is pale white to sickly gray, and their hair and beards are white, black, or orange. Their eyes glow with orange flame - one of many gifts from their fiery master, Ragnaros. ( In WoW they have darkish grey or black skin, with red glowing eyes. )
Dark Iron dwarves avoid straight combat, preferring ambushes and ranged assaults. Dark Irons are self-serving, with little of the clan loyalty so dominant in the other two dwarven clans. A Dark Iron willingly flees or betrays his allies in order to save his own life.  ( )
Dark Iron dwarves speak Common and Dwarven. Dark Iron dwarves sometimes learn Goblin and Low Common to speak with the creatures around their territory, and their elemental masters teach some of them their ancient language, Kalimag. ( )
Other Dark Iron Clans
With the presence of two other clans of dwarves with Dark Iron appearance, it is possible that the term Dark Iron refers to both an ethnic group of dwarves (such as hill or mountain dwarf) and the Dark Iron clan itself (which apparently lords over the other two lesser clans) or perhaps there is no single Dark Iron clan and instead several smaller clans ruled over by a single Dark Iron Emperor, who as such is lord over a confederacy of clans.
The Dark Iron dwarves bear much in in common with the duergar from Dungeons and Dragons, including appearance, habits, and origins.
Noteworthy Dark Irons