Arch-Demon Val'Gorath

A powerful Demon lord from a hellish alternative plane of existence, Val'Gorath sought to create his own dominion on the Prime Material Plane, resulting in a conflict which has become known as The Demon War, which raged from 1239 AoU until 1247 AoU, when Val'Gorath was defeated and banished from the Prime Material Plane by Crown Prince Aegic Enarren and his companions during The Battle of Auricheim.

A Cruel Twist Of Fate

On Midsummer's Eve, 1239 AoU, in the village of Mossbell, a young, talented, but untrained prospective magic user, named Gideon Alodar, sought to impress a girl with which he was in love, with the summoning of a friendly magical companion - a magical ritual which he had learned from a battered spellbook bought from a passing trader. His amateurish and hasty translation of runes beyond his understanding would result in the summoning, not of a benign arcane familiar, but of the bloodthirsty and evil demon - Val'Gorath, at the time a minor demonic lord in his home plane - into the couple's presence. Gideon's will would prove laughably powerless in containing Val'Gorath, and the demon would go on to slaughter every single inhabitant of Mossbell, save for Gideon himself; forcing the young man to watch as everyone he had ever loved met a grisly end at the demon's hands.
Perceiving his surroundings, and relishing the lack of bonds upon him, Val'Gorath quickly decided that he would make this plane his domain; visions of dominion, unncontested by other demon lords, setting his path in motion.  He would force young Gideon, his mind now broken, into his service as he began his plan of conquest.

Building The Horde

Conquest of the Prime Material Plane is not something that can be undertaken alone, even by a powerful entity such as Val'Gorath. Early on in his time on the prime plane, Val'Gorath would begin, at first through Gideon and later through those he recruited, begin to bring mortals under his thrall and into his service. An intelligent entity, Val'Gorath perceived and understood that to reveal himself too soon would bring destruction. It would be prudent to grow his influence and forces in the shadows, working to undermine the existing structures of society. The Cult of Val'Gorath was born, as his follower numbers swelled. Composed of mortals who swore themselves to his service, most completely willingly, the cult would spread throughout the lands of Vallanna and Ashalla in secret; their true allegiance kept from those who would oppose them.
Recruiting members to the cause wherever they went - outcasts, psychopaths, criminals and the dregs of society would form the backbone of the cult's forces, but to move in true secrecy and work to weaken the forces of order, often the cult would work to install their own members into positions of authority, or to influence those already in place, in order to destabilise the status quo, formenting a slowly growing atmosphere of chaos throughout the lands. 
As his minions went about their business, furthering his interests, Val'Gorath himself would begin to bring lesser demons - his servants from the hellish realms - into the prime material plane; building the army he would soon unleash upon the unsuspecting peoples of Vallanna and Ashalla.

Unleashing Hell

His forces growing, and his prospective enemies unaware and undermined, it would not be long before Val'Gorath would reveal himself. His demons and cultists unleashed, the divided peoples of the lands would find themselves facing a sudden invasion of horrific creatures and blood-crazed fanatics which they could never have prepared for. Entire communities were swept aside, put under siege, and cut off from one another. Chaos and death stalked the land; casualties among civilians were horrendous.
In the early years of this phase of the war, Val'Gorath's personal presence was often felt upon the battlefield; his cruelty and bloodlust leaving few survivors in his wake, and the legend of the demonic lord would spread, striking fear into the hearts of all mortals.

A Demonic Fortress

The war going decidedly his way, and the legions at his command swelling with the growing ranks of both cultists and minor demons alike, Val'Gorath would withdraw into The Auric Wastes, choosing the long-ruined Dwarven metropolis of Auricheim as his fortress. Deep within the mutating desert of the Wastes, Val'Gorath would transform the ruined city into a nightmare citadel, ascending a throne of bones in what was once the Royal Throne Room of the King of Auricheim, now profaned with his corrupting demonic essence.
It was upon this withdrawal into the Wastes that Val'Gorath would release his longest serving slave from his service - Gideon would take refuge far from the war in the mountain village of Smallfell. This would prove to be a mistake for Val'Gorath. Released from the hellish prison which Val'Gorath had made of his existence for nearly seven years, Gideon was able to regain something of his former self, and would go on to contact the Crown Prince, Aegic Enarren with details of Val'Gorath's plan to consolidate his forces in Auricheim. 

A Final Battle?

The actions of Prince Aegic during what would prove to be the final year of the war turned the tide against Val'Gorath's forces, and acting upon the information provided by Gideon, a large army was raised to trek into the wastes and assualt Auricheim itself. This attack would be known as The Battle of Auricheim, and it would ultimately spell doom for Val'Gorath's prospective demonic realm.
As fighting raged throughout the city, Prince Aegic and his heroic group of companions would force their way into Val'Gorath's throne room, engaging the Arch-Demon in combat. The fight would prove fatal for the young prince's mentor, Olarael Sin'ara, who would sacrifice herself so that the prince could strike a decisive blow to the Arch Demon.
Defeated, Val'Gorath was banished from the Prime Material Plane, and the day was won for the forces of good.
A demon of Val'Gorath's power is not so easily destroyed, however, and some say that he yet plots his revenge upon the mortals who stifled his plans and brought about his downfall, now over half a century ago.