In Warcraft canon, necromancy is simply the art of animating unliving flesh, and this can be achieved in several different ways. Anyone with magical aptitude is theoretically capable of practising necromancy, but it is considered taboo and highly illegal.
Since it deals with the manipulation of energies that are generally incompatible with life, those who practise enough of it can gradually become undead without needing to be killed.
When something is undead, it means that its soul is imperfectly attached to its body. The necromantic magic that powers them acts as a buffer that prevents their soul from properly rejoining with their bodies (as they would if they were fully resurrected). This imperfect link is why the undead may feel only faint sensations of pain or discomfort and why the Light is painful to them.
Death Knights and the necromancers of the Scourge utilise Death and Shadow magic, often combined with potent rune magic. Scourge necromancy has several forms:
Blood — drawing vitality, sustenance, and power out of blood and flesh
Frost — ice and "the chill of the grave", harnessing extreme cold and ice magic
Unholy — directly harnessing the most corruptive elements of necromancy and undeath; plague, pestilence, and animating the dead
Spirit magic — summoning, binding, and interacting with ghosts
Flesh magic — the practice of manipulating and reshaping flesh and the body
Bone magic — the practice of animating skeletons and manipulating bone
Domination — a particularly potent form of runic magic once unknowingly wielded by the Lich King through the Helm of Domination and Frostmourne; it has the power to completely suppress the will of an individual and corrupt them beyond recognition
Runes may be used in several ways, but Death Knights typically inscribe them upon powerful weapons to create runeblades. Runeblades are a form of living weapon linked to their wielder, empowering them while also instilling in them a hunger for pain and suffering. These weapons consume the souls of their victims.
Speaking of souls, they're pretty important. All living things have a soul, including worlds, and all souls can be corrupted.
It is possible to shatter a soul without completely destroying it. The cursed runeblade Frostmourne had the ability to shatter souls and did so to many, Arthas' included. Souls are known to be a highly potent power source across several disciplines.
NOTES ABOUT NECROMANCY
Since it deals with the manipulation of energies that are generally incompatible with life, those who practise enough of it can gradually become undead without needing to be killed.
When something is undead, it means that its soul is imperfectly attached to its body. The necromantic magic that powers them acts as a buffer that prevents their soul from properly rejoining with their bodies (as they would if they were fully resurrected). This imperfect link is why the undead may feel only faint sensations of pain or discomfort and why the Light is painful to them.
Death Knights and the necromancers of the Scourge utilise Death and Shadow magic, often combined with potent rune magic. Scourge necromancy has several forms:
Runes may be used in several ways, but Death Knights typically inscribe them upon powerful weapons to create runeblades. Runeblades are a form of living weapon linked to their wielder, empowering them while also instilling in them a hunger for pain and suffering. These weapons consume the souls of their victims.
Speaking of souls, they're pretty important. All living things have a soul, including worlds, and all souls can be corrupted.
It is possible to shatter a soul without completely destroying it. The cursed runeblade Frostmourne had the ability to shatter souls and did so to many, Arthas' included. Souls are known to be a highly potent power source across several disciplines.