Necromos has been a constant threat almost since the Vaxian Cataclysm.
Cataclysm and Youth
Necromos was a young human boy when the Kaa Taan destroyed the civilization that spanned the globe in the event that came to be known as the Cataclysm, some four thousand years ago or so. It isn't known whether Necromos was his real name, or a psuedonym, but probably the latter. Either way, he crawled out from the shattered ruins of his home and was forced to eat roaches, rats, and human corpses to survive for several years.
The young Necromos eventually made his way to the small college town of Tesaria where he became an apprentice. Sorting through the tumbledown ruins of the library there, he eventually stumbled upon a book called the Tome of the Nine Moons a set of notes and reference materials written by Averatos a powerful Necromancer (and possibly an arch-lich) who lived before the Cataclysm. Necromos studied these notes and gradually became a powerful Necromancer in his own right. Averatos's notes have since been lost, probably destroyed.
Necromos formed an alliance with various factions, most notably the Orcs of Ghuundagor and clashed with the Vaxian Empire under Kane Foraiya and Ramsen a number of times, never decisively. Eventually, when Minos was the Emperor and Necromos was a shriveled old man over a century old, they did battle over a number of the fabled Elemental Scepters, powerful pre-Cataclysm artifacts. At stake were the Scepter of Fire the Scepter of Water and the Scepter of Seven Thunders The Scepters of Fire and Thunder wound up in the control of the Empire, and the Scepter of Water wound up with Necromos, though both Necromos and Minos were killed.
But Necromos cheated: he had become so adept at Necromancy that he was able to single handedly claw himself back from the deepest levels of death, turning himself into an arch-lich, more powerful than ever. After his death and resurrection, he continued to bedevil the Empire off and on, and still does to this day.
Over time, he picked nine of the world's most powerful mages, who became his Apprentices of Necromos several of which are dead or forgotten, but some of which still live.
At some point, Necromos confronted and merged with the powerful life/chaos elemental Kosmo nullifying much of the arch-lich's power, and possibly splitting him into a number of separate fragments. It is not known which fragment has done what, or even if the fragmentation theory is even correct.
One of these fragments remained sequestered (possibly trapped) in a cave for a number of millennium. Augustus Razzaine and his friend Merguth visited this cave, and made some sort of deal with Necromos, and at least Razzaine became immune to magic as a result, while Merguth slowly went insane.
One part of Necromos masterminded the Darkest Night during which Ramsalon was devastated and almost destroyed. A few years later, he invaded Ramsalon again, fighting the Orc War which resulted in the destruction of Ghuundagor. This fragment is currently in hiding.
After Razzaine and Merguth vanished, Kosmo and the part of Necromos he had been merged with were somehow separated, and both are currently at large.
Necromos, aided by his Apprentices, worked tirelessly to adapt the remnants of pre-Cataclysm magic and technology to his own ends.
Working from Averatos's notes and some ideas stolen from Qua'ress, the Nethar goddess, Necromos created the Bloody Shroud Vampires Several of his Apprentices went on to modify this basic template, creating most of the known vampire clans.
Necromos adapted pre-Cataclysm portal technology to create the Underpass network, a series of portals which he used to ferry his troops around the world.
A collection of Necromos's notes and various arcane texts he accumulated is currently in the possession of the Empire. Access is strictly restricted.