The Legend of Idu Maagog and the Blade of Fire

Two and a half millennia before the Second Retreat of the Elves and the Dwarf-Giant Wars that saw the mutual decimation and fragmentation of both populations, the Age of Dragons ended and the Age of Magic began in the unquiet lands of the continent of Amarthaur.
Previously the sole domain of priests and druids, the scholar sages of Daozhong discovered and mastered the secrets of arcane magic, and, through trade and travel, brought that knowledge to the lands of Khoria, Zakhara and Thassilon.
In Thassilon this arcane knowledge brought about great transmutational wonders such as the Irespan of Magnimaros, as well as wondrous feats of enchantment and illusion; in Zakhara, powerful mages learnt to control elemental spirits and divine the future; but in Khoria, the peoples of Old Nuria and Yore turned their minds to darker arts of evocation, summoning and necromancy.
Whilst Elves had always been strongly affiliated to fey magic, the Dwarfs of Yore had been stubbornly pragmatic and resilient to such pursuits. However, as war broke out between the city states of Barcella, Tannesh and Akados (later to become Arcady), and spread south throughout the kingdoms of Old Nuria to become the devastation of the Arcane Wars, a Dwarf named Duorik Ironside, who had been instrumental in repulsing Giant incursions from the Icespires, and in overcoming Yeenoghu, a demonic Gnoll-Lord who had found a way to resist the Elder Gods, saw the power of arcane magic and learnt from Al-Qadim sorcerers to harness the Element of Earth.
With this magic he accomplished great tasks, including the banishment of the Dragon Queen, last of her kind, to a planar rift below the Stoneheart Mountains, and helped to carve out the great caverns of Runor far below the Halls of Hvela. He was renowned and celebrated amongst Dwarfs as a brave champion and saviour of their kind.
But his exploits attracted the attention of the Titan, Idu Maagog, an immortal Efreet who harboured a hatred of Dwarfkind that ran far deeper than competition over resources or land. Idu Maagog was a personification of rage itself, and this rage he channelled into a sword which he forged and cursed with bloodthirsty sentience. Employing the wiles of a rakshasa that was beholden to him, called Asuran, Idu Maagog gifted the sword to Duorik Ironside and watched as the tainted weapon gripped his soul and perverted his heart.
At first, Duorik used the sword justly to smite the Drow who had recently fallen under the dark spell of Lloth, but soon he was consumed by the rage of Idu Maagog and turned upon his own people, burning the Halls of Hvela in an inferno and forcing the survivors to either retreat further underground, where eventually they were transformed by fear and greed into Duergar, or to migrate west to the Ironcrags, Stonehearts and Trollheim.
Seeing what evil he had dealt out, Duorik killed himself and, tying his soul to the sword in a necromantic rite, swore he would not rest until Idu Maagog was vanquished and sent to Hell. In honour of his former glories, he was given a state funeral and buried in the crypts below Hvela along with the kings of old, and along with Flame Tongue, the cursed blade of Fire.

The Ages of Amarthaur

Age of Dragons (to c. 2,000 K.E.)
Age of Magic (c. 2,000 K.E. to 40 K.E.)
Age of New Gods (40 K.E. to present)