The Armies of Dread were a group founded soon after the Splintering by an Emissary of Cyrotix. The group was originally made up by a group of Necromancer Lords empowered by whatever might Cyrotix could conjure in the physical realm. They then set out across the realm and recruited dozens of necromancers. The Armies of Dread was the first time that the thought of using undead on such a wide scale as an army was first conceived. Sure undead were used as laborers or bodyguards, but they were more thought as as stepping stones on a necromancer's way to immortality. Combined with the power of Cyrotix channeled through the Emissary, you could easily raise an army of obedient, easily created, and steadfast soldiers. Thus the Age of Frost began.
The Armies of Dread quickly multiplied and increased because of Necromancy from the north, and soon enough they were large enough to destroy nations, which is what they did. No matter how many you cut down, more undead were ready to take their fallen comrade's place. Ignoring a few small problems, the Armies of Dread were on their way to conquer the entire northern half of the continent within months. Of course, that didn't happen as The Order devised the Coalition of Nations and planned to stop them. At first, with early skepticism, and the overwhelming forces of the Armies of Dread, the CoN got very little done, mostly just slowing down the great undead armies. But with a later deal with Pyrofaltic (long story), the CoN forces managed to push the Armies of Dread back to Death's Door, and there the Emissary of Cyrotix was struck down.
Afterwards, all the necromancers and Necromancer Lords (except one) were all hunted down and killed. After this event, necromancy became much rarer, and anyone who knew both necromancy and Cyromancy were likely to be killed.