My turn
Since noone seems to be responding, I'll post one of my own.
This idea came to me when I read The Ethereal Plane Handbook when it came out a few years back.
It mentioned that Illusions cast on the Ethereal Plane have a 5% chance of manifesting as a real entity/object.
In addition, there was a small chance that the entity would be diametrically opposed in alignment AND personality when it became conscious.
So, I had a theoretical question?
What if.... A High Level Lawful Good wizard cast an illusion of himself while on the Ethereal plane to escape one of the many beasties that wander that misty domain?
What if....that illusion became real, but was a chaotic evil version of the wizard (In addition, the original wizard was an anal retentive neat freak....this new guy would be wallowing in filth, revelling in all that is disgusting)
What if.... that CE wizard happens upon his own demiplane where he may concoct a devious plan of vengeance.
What if..... some time later, the wizard is killed by the evil twin, but is "survived" by a double created by a clone spell years earlier who has no memory at all of his Ethereal Plane incident.
What if....this clone, now much less powerful, enlists the heroes to track down his killer?
Yeah, I know the "Evil Twin" thing has been done to death, but this starts out as a locked room mystery which quickly turns into a plane walking epic.
I ran this a few years back, taking my PC's through all sorts of vile planes, including one of my favorites, Wormscape, a demiplane in the Ethereal Plane handbook: a plane made entirely of worms, with a few pockets of foul air that the PC's may attempt to travel through.
I had a lot of fun grossing them all out.
