Beyond the titular Latin-to-Arabic pun, ghouls embody gula (the cardinal sin of gluttony) in an obvious manner: they feed on the bodies of the dead. On top of that, ghouls are my favorite undead creature because, at least in their source material, they're not obviously undead. In Lovecraft's story Pickman's Model, it's implied that ghouls, like the inbred Martense clan in his less accomplished The Lurking Fear, are a degenerate strain of humanity, whose necrophagy forms part and parcel of their condition. Pickman even becomes a ghoul in The Dream-Quest of Unknown Kadath, where he and his meeping buddies come off as strangely sympathetic Addams Family types.
Although Gary Gygax made the conscious choice to make ghouls undead in going from Chainmail to D&D (more ghoul-trivia here), they seem like an obvious choice to explore as "voluntary undead" in this series. I've never quite understood how ghouls fit into the D&D undead mold of "if they kill you, they recruit you" because they seem so indiscriminately ravenous. A vampire might have the self-control to grant the "dark gift" of undeath or withhhold it, merely killing its victim ... but a ghoul holding back from a tasty, board-stiff human morsel? Hardly likely.
Living ghouls are gourmands and degenerates who develop a taste for human flesh and determine that the best way to get it is to feed upon the already dead. The diet awakens regressive tendencies in their genetic makeup and they become prognathous, robust and somewhat dog-like in their gait and demeanor. Haunting graveyards, they logically become followers of strong undead creatures, who feast on the souls of the unfortunate and leave ghouls the mere body.
Not being undead, living ghouls require a special reminder of their damned state in order to be effectively turned. In this case, it is sacred food, such as the consecrated bread or wine of Christianity, or barring that, foodstuffs created by a holy clerical spell. If the turning destroys the living ghoul, it lapses into a coma and can be returned to human state by forced feeding of consecrated food for three days.
The appearance of a ghoul - living or not - sets off ancestral memories in humans and related beings (dwarves, halflings, gnomes) but not elves, which can lead to one of the following fear effects - varying either by the individual, the ghoul pack, or on a racial level:
1. Save (Paralysis/Body) or paralysis by touch, as the original.
2. Howling causes a morale check among NPCs, and PCs must save (Spell/Mind) or become distracted, incurring -2 to hit and +2 to be hit.
3. Charnel smell within 30' forces a save (Poison/Body) or retch as with a Troglodyte's stench, unable to attack.
4. Paralyzing stare within 30'; victim gets two saves, one against the gaze (Spell/Mind) and one against the paralysis (Paralysis/Body).
5. Gibbering and meeping in close combat is maddening; requires a save (Spell/Mind) or become confused and attack a random adjacent being.
6. A hit from a ghoul's claws requires a save (Spell/Mind) or the victim cowers for a round, either fleeing or fighting defensively.
And no - I am not going to stat up the bizarre, half-medusa Roy Thomas comic-book version of Pickman's Model....