I'm currently running a 3.5ed Campaign for 7-9th level characters. The current trouble I have is that they're way too powerful combat-wise and I want to screw with their heads.
Here's what I've got so far:
I'm going to trap them in the town from 'The Created' (RM2). The mastermind for all of this will be a Mechanatrix Wizard of about 9th-12th level. He's going to be packing a lot of illusion type spells (minor image, major image, etc.) and going to have a lot of things floating around the town (invisible stalkers, doppelgangers, etc). He's a servant of the Xammux (Book of Vile Darkness) and he's basically torturing them in order to figure out how they tick.Does the party have any major nemesis or nemeses that this guy could somehow know about? Something they have been carrying that has been a scrying focus or mind probe? Then this Mech Wiz uses illusions of these foes...making them seem like they have come back with a vengence and teamed up against the party.
Another idea is to somehow split them up via dimension doors or something so that they each end up alone in the town and have to find their way back to each other. That might be tough to run for a little while one at a time, but no one likes being alone with danger lurking everywhere. Maybe this is where dopplegangers or other enemies tailored to the party members come in handy.
Would an invisible obscuring mist make monsters look partially transparent? Looks like "illusionary" monsters. <snip>
4. Reference to Spectral Force.
I could be mistaken, but I think 3rd edition no longer has Spectral Force, so you should change that reference to point to an existing 3rd edition spell, preferably one in the PHB.
No pointers balance-wise though. You'd have to playtest this to find out how powerful it is.

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.