The Empire of Bracaduun was one of the largest and most powerful nations in the history of Antagarich. Destroyed long before the events of Heroes of Might and Magic III: The Restoration of Erathia, it controlled the areas that would later become Bracada, Erathia, Krewlod, and Tatalia.
Bracaduun was ruled by the Wizard-Kings, a group of powerful wizards who used their knowledge of the mystical arts to dominate the continent. They enslaved the barbarians of the Wastelands and the gnolls and lizardmen of the Mudlands (later Krewlod and Tatalia, respectively), using them for manual labor and crushing any sign of rebellion.
After several centuries of brutal slavery, the barbarians rose up under King Tarnum, who had been inspired by the tales of the few surviving bards to throw off the shackles and fight their oppressors, hoping to restore the barbarian people to their former glory. After pushing the wizards out of the Wastelands and conquering the Mudlanders to increase his army, he made his way through the heavily defended region of the Wallpeaks and attacked Bracaduun, crushing their capital of Steelhorn and ending their long rule.
King Rion Gryphonheart, the founder of Erathia and first of the Gryphonheart line, was one of the knights in service to Bracaduun, and King Gavin Magnus, Immortal King of Bracada, was one of the defenders of Steelhorn - he discovered his immortality after being seemingly slain by one of Tarnum's behemoths.