The Inmaterial aka The Ectomorphs

A dungeon is often a place of pain and shattered dreams. (Well the body is shattered first, the quest for thee dream destroyed in the same process). This can make for the unquiet dead.

The ghosts of adventurers lost will often waunder the halls of a dungeon. They are not at rest for a variety of reasons, incompleted quests usually. They waunder the halls, often unable to effect the world. There presence adds to the ambiance of the place (dark, gloomy, cold, confused). Some will aid those adventurers who will complete their quests, while others hate those "heroes" who will succeed where they failed. This aid might simply be luck (or unluck), but more powerful spirits might give other aids that are within their powers.

The pain of death often disconnects a spirit from its normal path. These painful deaths often create spectres, poltergists, and other non material haunts. Many of these ectomorphs do not even know they are dead. In their half state, they simply see the living as a new kind of monster ("They are different from me, thus they are the enemy")

Either way, the inmaterial are usually disconnected from the reality of their situation. They refuse to see they are dead. They refuse to give up on their quest. They irrationally hate those who are living and succeeding, where they failed. Some can reasoned with, but others simply must be forced beyond.

Clerical prayers (spells) will help to remove these unquiet spirits - sending them along their way to their next existance. Some of these prayers are "common magic" or things that anyone who believes in the deity (lay person, acololyte or priest) can perform. However, the more powerful the spirit, the more clerical power required to remove it.

The Walking Dead

Zombies and Skeletons are often used to "stock" a dungeon. Death by any of these creatures often is "viral", causing the dead to rise as a zombie or eventual skeleton. The body is no longer part of the physical food chain, except as a mechanism for additional death (causing food for the carrion here). These creatures normally attack intelligent being, but some will just attack anything that appears to be "alive". It depends if the walking dead were created naturally (through hate and evil and spiritual bindings) or made (by some necromantic or religious cursing).

The spirit is free to pass while the body is animated. Only a few spirits will care what happens to their human carcass when they move on.

It is not all death and magic. There are molds and worms that are symbiotic with the various undeads. They are the living part of the undead cycle.

Vampires
This is another form of Viral Undead. Victims of these creatures become minions of the creature. Their souls are bound into their forms... the vampiric ectomorph nesting in it... while manipulating the body. These creatures are bound to strike the living. Though they are not part of dungeons proper, dungeon like spaces are often used as lairs for such creatures.

These creatures can be "destroyed" or "blessed" driving the vampiric ectomoph back to the negative existance. This usually releases the spirit, but sometimes the vampiric spirit holds on to it for power.

Demons and Dark Spirits

Let us face it, most dungeons are not "nice places". Dark Forces pool in hidden places, away from the prying eyes and The Light. What is more hidden than a dungeon?

Demons were often summoned by those who created the dungeon. They were used as sentinels and minion of the creator. (Though, some use them as builders as well.) These demons have specific goals they were charged with when summoned. Now, because they are demons, they probably have their own goals... the main one being trying to work around the charged orders.

Now free floating Demons, those that just waundered to the material realm and made the dungeon their home. These just want to create mischief and to defend their "home".

Now once demons are involved, the course of ones's soul - once seperated from the body - comes into question. They might "snack" upon those they kill or injure in the dungeon. More clerical prayers might be required to ensure the proper movement of souls after death.

Demons are normally must be delt with materially or with magical abilities... to discorporate the demons from the material plane. Demons are often removable (or controllable) by clerical prayers.