In Pannithor, the dead do not rest easy. The long dead warriors of ancient battles still lie beneath the soil of the world, these thousands of these skeletal remains left without burial or respect, forgotten into time. Yet it is these remains that are perfect fodder for a necromancer to create legions of undead soldiers.
Once a necromancer reaches sufficient power to begin putting their dark arts into practise, they risk discovery and thus a death sentence. It is hard to disguise necromantic magic, for it is indiscriminate. When a sorcerer weaves the incantations of awakening, it will re-animate every dead creature nearby. In such a manner, the decomposing bodies of dogs, rats, birds and horses will inevitably be raised along with the corpses of warriors and long-dead heroes. All are enslaved to the will of the necromancer, but all increase the chance of some do-gooder discovering the necromancer’s foul experiments.
– lore snippet from 3rd Ed. Rulebook