Well, this is an idea I have planned for my next game, and it's not ghoulish, but you might be able to adapt it.
I have an order of monk/psions (egoists) who gain their powers by briefly drawing on the soul of another creature. Training begins by learning how to tap into the physical soul which is bound up with the body. The monastery keeps caged bears, hawks, rats, and other standard creatures, and so some of the primary abilities the psions take are things like Claws of the Bear, Animal Affinity, Featherfall, and a few other shapechange abilities. More powerful members of the order can tap into more intellectual aspects of the soul, allowing them to gain skill ranks, feats, and also knowledge (a la telepathy).
When members of the order abuse their powers, they are cast out, and their souls are rended, leaving a soulless body with a mind, but no compassion. It is possible to recover your soul, however, and one of the PCs is going to try to do just that.
In the monastery, he'll have to pass several tests to prove his spirit, and there'll be some psychological delving, but the climax will come when he and the rest of the party come upon the last member who was cast out of the order. When this guy tried to pass the test, he came with his companions, but he failed the test, and in the process the group of them all lost the ability to pull their souls apart, and they created an amalgam of each person's body.
The creature resembles a giant head, with one large face on the front that is twisted in pain. Instead of a normal mouth, though, a dozen arms thrust out at different angles. Around the sides of the head are over a half-dozen other faces, all distinct as if they had been individuals before they became a piece of this monstrosity. The head floats in the air, the air around it filled with a cacophony of cries, songs, screams, and moans.
Where the giant head's ear should be, the shoulders, head, and one arm of a woman thust out, and she beckons for the group to approach, singing a lament. Near beside her is a face with flesh drawn tight across the skull, screaming in agony, its eyes rolled back. The other faces consist of an Elf woman who continually claws at the body around her with the one arm she can still control, a reptilian head that is half-caught in the flesh and puffs flame from its lips, a dark Elf from whose mouth spiders scramble and drop to the floor, an elderly man with a tear-streaked face, and the lolling head and neck of a blank-eyed woman. There might be more, but you can't see them clearly.
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I basically needed a monster, and always wanted to use a nice beholder, but with a twist. The monastery is called the Temple of Echoed Souls, so I decided to make a sound-based Beholder, with various effects caused by song and screams. The arms can tear a person apart much like a bite attack could, and the central face still creates antimagic, but I shuffled the powers around.
A floating beholder-esque clump of ghoulish spellcasters might be a possibility. Each one could cast a different spell, or even have it's own complete spell list.
WHY didn't I think of the ballistae idea myself? And the undead arrow idea is superb. I'll just stow that away for future use. But I don't think you'll be disappointed anyways! We'll find out in a few days.
