Following the end of the Mage Wars, Necromancers proliferated throughout the Known Worlds. Since the wars had ended, their primary role as mercenaries was no longer profitable, and they began to turn to other industries.
Though temples tried to be self-sufficient, they often needed support from surrounding communities, to which they sold their services for goods and money. Cheap labor was common, as a dozen necromancers could direct several hundred reanimated corpses at simple tasks. Mining and farm work were the main sources.
The war's primary instigator was the Grey Temple, who felt that the world belonged to them and they should take it. A schism between Grey Temple and Green Temple necromancers at the begining of the Third Age had caused nearly all temples to identify with one of the two theologies.
However, while Grey Temple theology included a sense of entitlement, the Grey Temple necromancers lacked any support system. It took the rise of Arch-Lich Honoreck to unite the Grey Temples and begin a campaign to take over the known worlds.
The bulk of actual fighting in the Necromanic Wars was done by the Grey Temple necromancers, who by this time had amassed several hundred Liches. They had many allies who commited living troops to the cause, and within a few years had captured a great deal of territory.
Honoreck had promised his followers a thousand-year nation forged in war, and many were eager to oblige. Those that would not fight willingly were executed, and their corpses made to fight for the cause.
In total, the war lasted for 108 years, but claimed more lives than the Second Chaotic Period, by reason of Honoreck's tactics which placed almost every living fighter neck-deep in rotting reanimated corpses.
Honoreck's thousand-year nation came to an early end when the Slayer Dragons forged an alliance with the Green Temple (which, until then, had largely stayed out of the whole debacle). The Slayer Dragons brought in huge armies of trained and well-equipped living soldiers, and using Eieber's playbook, brought about a quick and decisive end.
The main technique involved Green Temple magic to turn Honoreck's own armies against him. With relatively few human troops on the battlefield, this tactic was brutally successful. Honoreck himself was slain by Lal Soratami, who would later become Pendragon in A.Y. 3411.
The real tragedy of the Necromanic Wars was that both sides actively employed large numbers of undead, which meant festering corpses everywhere, spreading disease, whereof more people died and joined the undead armies than were actually slain.