Here's a thought - send any high-level goody two-shoes adventuring party they might have come up against (that survived, that is) after them as rivals. Let them encounter each other multiple times throughout the adventure, splitting them up with well-times teleports each time before anybody gets killed, leading up to a climactic encounter at the final site.
You guys with high level PCs should check the old 2e adventure "Nightmare Keep".
It's high level hell!!!
From what I can remember one of the first encounters inside killed 3 out of 4 characters(1 fighter, 1 Priest and 1 wizard, the other wizard survived) in 2 or 3 rounds. It was 2e, granted, but the characters were all 18th level...
"Misty morning Clouds in the sky Without warning A wizard walks by"