Ghouls being an undead creature has never rhymed well for me. My take has always been more in tune with ghouls as presented in GURPS: Fantasy Creatures, that is as a living race which feasts upon the dead. My ghouls live in or near cemeteries for ravenous need. Their lairs are crypts or barrows hidden in or near cemeteries.
The following is presented under the Open Gaming License. All rules quoted are from The Basic Fantasy Role-Playing Game copyrighted by Chris Gonnerman. It is easily adapted to your favorite fantasy role-playing game of choice.
"Ghoulism" is a disease, mystical in nature. Most often, ghoulism infects necromancers, or any other living creature that is obsessed with death. Those in the first stage of infection often find themselves panting and sweating in bed after having a bout of night terrors. These night terrors often involve dreams in which the infected is feasting upon corpses. Slowly, the infected finds that she gains no sustenance from typical foods and after a period of a few days, typical foods cause severe stomach cramps and there is yearning for dead flesh. The sight of a corpse, fresh or otherwise, will cause salivation. The smell is invigorating. There is hope for a cure at this stage. A Cure Disease, cast immediately before a Bless by a Cleric of at least 6th level will cure the infected. Being anointed with Holy Water will not cure, but it will help ease suffering (+1 on Save vs Paralysis or Petrify, see below). The casting of a Remove Curse by a cleric of 9th level or higher will also restore the infected to health.
Physically, in the first stage, the infected becomes gaunt and cadaverous in appearance. An odor of rotting flesh is an undercurrent of vile perfume radiating from her body. Within a few days, her gums blacken and soon her teeth begin to fall out. The next day the small pointed tips of jagged, razor like canines begin to peek through her blackened gums. Shortly, she notices that her nails are elongating, becoming hard and pointed.
After being cured at the first stage of the disease, the character slowly regains her lost constitution points, and with time, her appearance reverts to normal; although, she may retain a gaunt appearance as a reminder of what almost was.
If the infected is not cured within the first week of contraction, then a Save vs Paralysis or Petrify must be made daily. Failure results in a constitution drop of one point per day. If constitution drops below half, or to five (whichever is lower) the infected becomes a ghoul. If at any time during the first stage the infected gives into her yearnings, the change is automatic. At this point, the mystical disease can only be removed with the casting of a Remove Curse by a cleric of 9th level or higher, an unwilling ghoul may save versus Spell to resist. Any constitution points lost as a result of the first stage are now regained in the second stage.
Now her appearance is definitely ghoulish. Her former teeth are all gone, replaced by a jagged row of razor sharp canines meant for rending flesh. The hands she views at her wrists end in only what can be described as talons. Her eyes become a dull yellow, and her tongue is a black, elongated slug. It is possible for her to hide her appearance, but not without the proper spells. The smell of death and decay issues strongly from her pores. She tries to hide this with strong perfume, but now she smells of death and lilacs.
Curing her at this point in the disease, her constitution automatically drops to half (or five, whichever is higher); however, this is not a permanent reduction. She will gain them back normally, minus 1 point for every month spent in the second stage. If she was afflicted in the second stage for a year or more, then the loss is permanent, short of a Restoration being cast upon her. She will always maintain a gaunt and slightly cadaverous appearance, and her moldering smell will be disquieting.
As ghoulism is normally caught by those traveling the path of necromancer, it is not uncommon for ghouls to be magic-users, and also rarely clerics of a death god. In this case, the ghoul is treated as an NPC of appropriate class and level (typically 4th, but ghouls of higher or lower levels are possible). Even ghouls in the second stage of ghoulism that are not leveled NPC's with a character class have the ability to cast Speak with Dead as a cleric of 6th level. This is accomplished by the ghoul at least partially eating the corpse, after which the ghoul may enter into a sort of hypnotic state. After 1 to 3 rounds (depending upon how much of the corpse the ghoul ate, more equals less), the corpse will answer questions per the spell description of the same name. This ability makes ghouls sometimes sought after by adventurers and savants.
Ghoul characters often accept their fate at this point. Some form alliances with mages, death cults and even thieves guilds. It should be noted that while their claws and teeth are usable as weapons, for the most part ghouls attack with weapons and prefer missile weapons versus moving in close. They are not combatants and will avoid it if possible with the use of guile and spells, if available.
The third stage of ghoulism is more comparable to the standard ghoul description as described in most D&D type monster manuals. It is not known why the ghoul eventually devolves to this state, nor is there a set pattern on how soon it will happen, but eventually the madness takes the infected down the last road of depravity. At this stage, the ghoul is a raving monster. Ghouls in the second stage of the disease will hunt down those in the third stage and destroy them. This is done out of self-protection. Ghoul societies are by necessity small and secretive. A raving ghoul brings unwanted attention. Subsequently, Ghouls in the second stage of the disease will also seek out those in the first stage and most often destroy them for the same reason; however, there are rare cases of mercy being taken upon these souls and they might be inducted into the ghoul society after they ease them into the second stage by serving them their first feast. Ghouls who reach the third stage cannot be cured of the mystical disease. They are lost and their only salvation is in death.
The ghoul in the third stage appears much as the second, but her mind is gone. The mold and mud of the graveyard is no longer removed from her clothing. No attempt is made to hide what she is.
Ghoulism may also be contracted from the bite of a ghoul; fortunately, only those ghouls in the third stage would risk spreading ghoulism in this fashion (although a confused infected in the first stage might as well). In addition to saving versus paralyzation to avoid the standard ghoul attack, if a character is bitten, a second save must be made versus paralysis to avoid contracting the disease. This is only for bites. Elves are immune to the paralysis of the ghoul and are immune to the disease. There are rare cases of cannibals contracting ghoulism by eating "tainted" meat (a corpse infected with ghoulism).
As ghouls are not undead, they can not properly be turned; however, a successful turn attempt by a cleric of 1st to 11th level will cause the ghoul to pause for a round (treat the ghoul as "surprised" for combat). This result is automatic for clerics of 12th to 14th level. Clerics of 15th to 20th level do not "destroy" the ghoul (as per normal), but the ghoul will attempt to flee from the cleric's awesome display of faith. It is also possible for a cleric of 15th or higher level to be allowed to command a ghoul, if the cleric is of a sufficiently evil bent. Take note that leveled and classed ghouls are not so easily turned. A classed ghoul of 3rd level is treated as a Wight, a 4th as a Wraith, a 5th as a Mummy, a 6th as a Specter, 7th or higher as a Vampire. The above effects for turning are used as level appropriate (i.e. a 5th level Magic-user Ghoul: a 1st to 5th level Cleric has no effect, a 6th to 17th level cleric may "pause" the ghoul, while an 18th to 20th level may cause the ghoul to run away in fear. A ghoul of this level could not be commanded).
Other than those items noted above, the ghoul stats from your favorite monster listing is used, with the "standard" ghoul from said book standing as a ghoul in the third stage of ghoulism. Those in the first stage would have their normal attributes/statistics, and please take note; ghoulism is not regulated to just humans. There can be orc-ghouls, ghouls, hobgoblin-ghouls, or even kobold-ghouls (twisted, and a bit funny, but go for it), just not elf ghouls (unless you really don't like elves and wish to ignore said rule--see rule #1 DM Fiat); hence, a ghoul in the first or second stage could have all sorts of wacky attributes/statistics. If you allow monsters to have classes in your campaigns, things could get really interesting. Consider a 3rd level Cleric, Lizard Man Ghoul. Undead should not be ghouls. It's your call if Lycanthropes, but it is my view that one mystical disease is probably enough.
There is a reason that elves are immune to ghoulism, or at least the legend of a reason. It seems the first ghouls were the product of an experiment conducted by elves. Many elves, deny it. Some of them say it was the drow