Blackrock Depths

Blackrock Depths (aka Blackrock Deeps) is the deepest part of Blackrock Mountain, ruled by the Dark Iron dwarves who are led by Emperor Dagran Thaurissan. Several minions of the Firelord Ragnaros also make their home in these lava-lined depths.

History
From Blizzard's Official World Dungeons site:
 * Once the capital city of the Dark Iron dwarves, this volcanic labyrinth now serves as the seat of power for Ragnaros the Firelord. Ragnaros has uncovered the secret to creating life from stone and plans to build an army of unstoppable golems to aid him in conquering the whole of Blackrock Mountain. Obsessed with defeatingNefarian and his draconic minions, Ragnaros will go to any extreme to achieve final victory.

The volcanic Blackrock Depths are immense tunnels beneath Blackrock Mountain, the home of the Dark Iron dwarves, and the site of the black forge they use to smelt their namesake ore. The Dark Iron dwarves have claimed the depths under the banner of their emperor, Dagran Thaurissan. Thaurissan is the latest in a line of dwarven sorcerer-thanes to serve in the shadow of the demigod Ragnaros, foolishly summoned to Azeroth by Thaurissan's ancestors hundreds of years ago. Though the dwarves dominate Blackrock Depths, they worship and labor for Ragnaros's mighty fire elementals, who guide the already ill-tempered and maligned Dark Irons towards increasingly dangerous and apocalyptic acts.[1]

Dungeon Journal
''The smoldering Blackrock Depths are home to the Dark Iron dwarves and their emperor, Dagran Thaurissan. Like his predecessors, he serves under the iron rule of Ragnaros the Firelord, a merciless being summoned into the world centuries ago. The presence of chaotic elementals has attracted Twilight's Hammer cultists to the mountain domain. Along with Ragnaros' servants, they have pushed the dwarves toward increasingly destructive ends that could soon spell doom for all of Azeroth.''