What I would like to discuss are these:
Extract Gift Spell:
A 5th level necromantic spell that more or less does this: Gets a demon and a willing creature and gives the creature a portion of the demon's essence.
A demon with 18 str can "gift" that way a creature with a +2 to strength or a demon with 10 ranks in bluff can "gift" with a +5 bluff. Enhancement bonuses all these.
Takes an hour and as much XP and gold as crafting an item that does this. Dispellable like permanency. So to give a +2 to strength the caster must spend 4000 gold in paints incence etc and 160 XP. Both wizard and cleric spell.
The demon needs not to be willing for the spell to work but 1. can resist with a will if not, 2. needs to be present for the casting. Planar binding works fine.
So? What should the modifications be in Ravenloft? Powers checks for one thing. Anything else? Ideas?
IMO the spell should be more risky and give more rewards.
Demonic possession: By the book some demons can possess items. While having a demon possessing a fountain placing curses on whoever drinks it sounds fun, what I really liked is this:
A demon can make a normal weapon/armor function as if magical. However the weapon doesn't register as magical, only as evil (doesn't work in Ravenloft of course).
A 9HD demon can imbue an item to be worth 18000. So a hezrou could take over a mundane (not even mw) sword and change it to a +1, unholy sword for example, or a +2 keen sword etc.
From the sword, the demon can whisper to his owner and do many nifty things quite Ravenloftish.
You can say "Hey, a intelligent evil magic item works quite the same fool!" but here is one more quirk:
A character (even a player character) can planar bind a weak demon with 5 HD (thus turning a normal armor to a +3 armor for example) at 9th level. Sure, he can safely only do so for 9 days, but that would be more than enough. After all, he won't keep the demon-bound armor long enough to be tainted, would he? The demon is weak enough probably that a party of 9th level chars can take him down easily.
It doesn't even has to be out of Ravenloft. The party knows one such fiend from a musty tome they found. So no harm done right?
PS. A lowly imp or quasit with this ability has enough power to turn a +1 sword to +2. The PCs really need to improve their +1 rapier to ghost touch? "Just to deal with that pesky specters that terrorize the villagers and make more of their own each night" perhaps?
Other option: What about a foolish mage that summons and binds three 9HD demons with this ability for a year? He has equipped his most trusted minions with +1 unholy weapons out of the blue in 2-3 days. Not only the PCs have to face suddenly a group of well armed evil indivifuals with demons whispering to them, but they also need the wizard alive to give them the fiends' bodies or at least a clue about the fiends' names etc so they can deal with them.
They should be warned that if they kill the wizard, the demons will bw free to do as they please, probably taking back their bodies and wrecking havoc.