Undead
Sleep thought from the other day: vampire native form is black mist. They possess bodies which decay at normal rate. Must suck blood to keep body fresh. More blood = younger and stronger
Some games have vampires possess living human bodies.
I like corpses because they have a shelf life, dissolve into goo, and have an excuse to force vamp to drink blood
Also image of vamp w/o fangs biting & sucking blood like a remora is fun
They can't cross running water because it'll wash their mist body out thru orifices
Also they have to return to grave soil at morning to refresh. Store your rotting meat suit in a coffin of vermicompost. Back up bodies in giant urns of fermented sludge.
Raising dead requires installing a new soul. There's different ways of doing this but jewels are compact and effective. The more valuable the jewel the stronger the soul it can contain. Djinn and Efreeti are basically incorporeal so they can be captured too by a sufficiently potent sorcerer.
Easiest source of souls is captured children, but they're the least competent. Thus skeletons sing nursery rhymes, play hopscotch, pelt PCs with stones (harmless but annoying), and run away cackling when they're caught.
Adult souls are more capable but they can be confused by death, especially if they died unexpectedly or in great anguish. Unless regularly subdued they eventually go mad and turn feral.
Unwilling souls lose their memories, will, and the majority of their prowess, though they may retain bits. Thus undead servants can be raised to perform menial tasks based on their living profession.
Souls of scholars can organize libraries, sort texts, and act as scribes, though they're mindless and incapable of understanding any of it, being basically automata, so they can't combine ideas or interpret materials.
Souls of warriors make okay fighters, but fall short of living warriors. The benefit is that they're fearless, never tire, and feel no pain.
Undead peasants, porters, sailors, and smiths can pull plows, tie down cargo, adjust sails, and produce works, but their quality is shoddy at best. They don't tire or require supervision though, although they don't know to switch tasks on their own so you're liable to return to the forge and find a thousand horse shoes and a bored skeleton counting ants in the corner.
Willing participants are best and rarest, they maintain their memories and capabilities in death but their personality is erased. Never possess a corpse with it's original soul. Just trust me on this.
Fragments of personality may occasionally bubble to the surface. Nothing important. They may start imitating past activities in a harmlessly mindless way, organizing snail shells by color and size, staring at stars, counting hair follicles on a scalp, staring uncomprehendingly at a page in a book, or playing a game of mock chess with themselves using rocks. All done totally silently. Without orders they'll wait around patiently until something triggers them.
Skeletons have the soul crystal imbedded in their skull, the cavity of which is filled with gelatinous ichor. The top of the skull is replaced with riveted steel. Many necromancers keep desiccated mummies around as backstock. They're nice because they don't fall apart or rot.
Shamblers are walking corpses and have soul crystals embedded in their exposed hearts which swell and pump black ichor through their veins, animating them. They decompose in time if not taken care of, albeit slowly. If kept in a cool dry environment they can last centuries.
All undead go feral if their master dies, if they aren't given orders for too long, or if the incantations subduing them aren't refreshed often enough.
Souls trapped in undead bodies suffer great agony. They don't know what's happening to them and they seem to be living in a hellish prison and tormented by terrible visions mixing their past and current unlife. The suffering caused generates serious karmic defilements in the caster, twisting their karmic alignment towards chaos. The rebirths of powerful necromancers are invariably in the worst hells.
Thus they are motivated by self preservation to find the secrets of eternal life, to cheat reincarnation, or to transmit their souls to demons or other necromancers on the promise of safe keeping or installing into a fresh body. The worst kinds of lich are a sorcerer and apprentice pair that pass their souls back and forth for eternity, cloning or capturing new bodies with each iteration, and growing forever in knowledge.
Wraiths, ghosts, shades, shadows, specters, voidlings, etc are confused dead souls which, due to unsatisfactory burial, incompleted obsessions, or other types of clinging failed to successfully transfer into the Mouth of Death. Without realizing the nature of their state dead souls often fixate on their old life and become trapped on this plane when the Mouth of Death gate closes.
Thus they wander, trying to live their old life, slowly going insane with frustration. Often they become violent or vengeful. These spirits can be manipulated by evil sorcerers, used as slaves or guardians, or sent as agents to inflict disease and misfortune.
All undead feed on life essence, often in the form of spiritual essence, flesh, organs or bodily fluids.
For information on the nature of the afterlife, attachment, and rebirth, read "the Tibetan Book of the Dead". I recommend the Jung commentary. I'll do another segment on how that plays into my campaign world.