Pick up Underdark, and check out the rather small Howling Warrens section.
It's a 4e version of the 3e and earlier Pandemonium plane.
step 1 ... -5 to Perception checks to determine if the PCs can hear the howling
step 2 ... notice the disease has a level and a plus (!)
step 3 ... notice the save value goes up by +2 each time
result ... it is entirely possible that the PCs never
know they've actually wandered
into The Howling Warrens. The save number keep ratcheting up, to unbeatable.
And the disease level can go up too, also to unbeatable levels. The PCs have to
blow all sorts of coin and time and rituals, to figure out what's happening. Too late.
Encounter powers become 1/day. Dailies are also just 1/day.
Huzzah --- says the mean-o