Necromancy is a school of magic founded by Xalthar, the First Necromancer. Xalthar had the secrets of necromancy revealed to her by the Icosahedron, and was quickly consumed by the power that it offered. Eager to expand her newfound shambling empire, Xalthar taught others the secrets of her craft to act as lieutenants and servants to carry out her will. Thus, even long after her defeat and torture at the hands of Harika, Xalthar's knowledge spread through Creation and necromancy found its way to other lands.
While various forms of undead roam Creation, those created by necromancy are referred to as dregs. Dregs are reanimated through dark rituals and incantations that were taught by Xalthar, who in turn had knowledge of them revealed to her by the Icosahedron. Thus, the inner workings and origins of these rituals are not entirely known. The end result appears to be shambling corpses that can be controlled to some degree, but have a propensity for attacking anything living if left unchecked. Dregs that belong to apprentices usually deteriorate and fall apart within a few days, while those of more accomplished necromancers can seemingly wander without their masters indefinitely.