This particular question wasn’t online – but it is an interesting one, even if it has been addressed before in various places.
In basic d20, do Undead have Souls?
Well, since it’s basic d20, lets see what we can get from the System Reference Document.
As far as Creating Undead goes… Create Undead requires “a clay pot filled with grave dirt and another filled with brackish water. The spell must be cast on a dead body. You must place a black onyx gem worth at least 50 gp per HD of the undead to be created into the mouth or eye socket of each corpse. The magic of the spell turns these gems into worthless shells.”
Evidently “Create Undead” doesn’t require access to the soul even when it’s creating intelligent undead or templated undead. This fits in with the general rules about death and souls. You can find those under “Bringing Back the Dead”
Several spells have the power to restore slain characters to life.
- When a living creature dies, its soul departs its body, leaves the Material Plane, travels through the Astral Plane, and goes to abide on the plane where the creature’s deity resides. If the creature did not worship a deity, its soul departs to the plane corresponding to its alignment. Bringing someone back from the dead means retrieving his or her soul and returning it to his or her body.
- Level Loss: Any creature brought back to life usually loses one level of experience. The character’s new XP total is midway between the minimum needed for his or her new (reduced) level and the minimum needed for the next one. If the character was 1st level at the time of death, he or she loses 2 points of Constitution instead of losing a level.
- This level loss or Constitution loss cannot be repaired by any mortal means, even wish or miracle. A revived character can regain a lost level by earning XP through further adventuring. A revived character who was 1st level at the time of death can regain lost points of Constitution by improving his or her Constitution score when he or she attains a level that allows an ability score increase.
- Preventing Revivification: Enemies can take steps to make it more difficult for a character to be returned from the dead. Keeping the body prevents others from using raise dead or resurrection to restore the slain character to life. Casting trap the soul prevents any sort of revivification unless the soul is first released.
- Revivification against One’s Will: A soul cannot be returned to life if it does not wish to be. A soul knows the name, alignment, and patron deity (if any) of the character attempting to revive it and may refuse to return on that basis.
Obviously this wasn’t checked during the 3.5 update. 3.5 allows Resurrection without the body and has changed Trap the Soul so that it entraps the body as well. Fortunately, this doesn’t change the point.
The SRD also provides us with a definition of Death and what happens there:
A character dies when his or her hit points drop to -10 or lower. A character also dies when his or her Constitution drops to 0, and certain spells or effects (such as failing a Fortitude save against massive damage) can also kill a character outright. Death causes the character’s soul to leave the body and journey to an Outer Plane. Dead characters cannot benefit from normal or magical healing, but they can be restored to life via magic. A dead body decays normally unless magically preserved, but magic that restores a dead character to life also restores the body either to full health or to its condition at the time of death (depending on the spell or device). Either way, resurrected characters need not worry about rigor mortis, decomposition, and other conditions that affect dead bodies.
So; Resurrection – a ninth level spell – can’t affect a body that’s been dead more than ten years per level and cannot affect an unwilling soul. Per the rules above, no spell, however epic, can be designed that bypasses that restriction. Animate Dead or Create Undead – at levels three and six respectively – can affect bodies that have been dead an unlimited length of time, even if the soul of such a body has risen to become an archdemon, or achieved godhood or some such.
While that does open up the amusing possibility of destroying gods and demons and devils and such who rose to that position after dying as mortals (the standard d20 start for new demons and devils) by finding their original bodies and turning them into mummies or something, the implications change the setting quite a lot and certainly does not fit in with the rules. If a spell can drag an unwilling soul back and stuff it into an undead body, why not into a living one?
Thus we can deduce that undead do NOT necessarily have the original soul unless, perhaps, it was willing – or possibly unless it went straight from being alive to being undead. Instead they usually have some sort of artificial mirror-image of that soul, created from negative energy. That would, of course, explain their hostility towards everything the original soul loved, makes sense of the workings of the various undead-creating spells, keeps people from killing gods with third-level spells, keeps souls from being recalled from the planes of good and forced to change their alignments, and still explains why you can’t resurrect people who’s bodies are currently active as undead; a soul and it’s mirror-image cannot exist simultaneously in the inner planes.
Souls that want to become undead may or may not be able to “answer the call” and take up unlife – but I’d say that they can. That makes sense out of liches and voluntary vampires and such.
Turning someone directly into an undead without allowing the soul to depart may or may not bind the soul. It probably doesn’t though – first because it’s pretty hard to go from “positive energy” to “negative energy” without passing through “zero energy” (dead) along the way and partially because trapping souls is supposed to be REALLY high level magic again – something that it doesn’t seem likely that minor undead are capable of with simple draining attacks. Save it for those high-level mages voluntarily turning themselves into liches and accused death knights returning from the lower planes as undead and such.
This also answers whether or not you can make a sapient undead out of the body of someone who’s transferred to a Clone (something the original rules do not address). By this analysis, no you can’t; the original, positive-energy aligned, soul is still present on the inner planes, so such an attempt will automatically fail.
Spells and abilities that prevent corpses from being turned into undead are thus pretty simple; they keep the body from being effectively infused with negative energy. That’s why simple, low-level, effects work just fine; all they’re doing is purging the remnants left over from life which would have gone into forging the quasi-soul. Arguably that might not actually keep some necromancer from turning the corpse into a skeleton or zombie – but it very well might. After all, with nothing left to set the pattern, infusing a body with negative energy won’t work any better than infusing a random rock with it.
Now, many game masters may run variant worlds where undead (at least those who go beyond being simply automatons like skeletons and zombies) DO get the original soul all the time – but they should be careful about the implications that has. In that case it’s probably best to say that undead can only arise when the soul involved consents. That would help keep the world from being overrun with self-propagating undead like Shadows too.