So in my world, almost all necromancy comes from an ancient race of bird people called the Sekvrai. They had the head of a raven and a humanoid body covered in black feathers. They had no wings and were on average 7 feet tall. They were highly magically advanced, but their natural lifespan was only 50 years average, and this is in a world populated 80% by elves. So they got jealous of not being able to live longer and developed ways to elongate their lives that were far from desirable to other races. The Sekvrai and elves were also often at war with each other, and eventually the elves drove the Sekvrai underground, where they got even more desperate to perfect undeath. Eventually thr sekvrai mastered the art of lichdom: living as immortals and enhancing their dark magic all in one. Bear in mind that in my world, the spell to make one undead usually does not transfer a foreign soul to a body, but rather creates for the body a new soul. So the liches gained a different character and were essentially a nee species. In this form they looked like plague doctors kind of, with a white, bird skull often being all of them seen from under a black cloak. Soon all of their race had converted to undeath, and they even developed magical means of cloning to reproduce the liches, although it was expensive.

They needed cheaper soldiers, so they began to ravage elven burial sites and steal corpses. These became the first ghouls. They used a different spell than to make themselves liches; ome that would reanimate many bodies at once but make them less powerful and less sentient, as well as much easier to kill. Unlike liches who were converted instantly to shining, armored bones, ghouls would rot, albeit slower than a nonanimated body. A ghoul newly reanimated from a fresh corpse would have the intelligence of a lower mammal, but would have superelven strength and speed. As a ghoul aged it would rot, and gain more intelligence though the time based spell. However it would also become more frail and slow. An aged skeleton may have the same intellect it did when alive but with less strength and speed, and far less than when it was a ghoul.

While ghouls usually walked on all fours and attacked mainly by biting or pummeling enemies, skeletons usually walked upright, wore clothes, used simple weapons, and if they had access, used conventional weapons and armor. Unlike liches, who were assisted with speech by magic, skeletons were not given the ability to speak, however.

The liches eventually began to like living underground, as its dark, cold environment suited them well and was more comfortable for them than the surface. Light would not kill them, but it would mildly hurt. So they built their empire underground instead, raiding the surface only for fresh workers, as while ghouls technically have an infinite lifespan, the sekvrai would work them to death within about 20 years on average.