coyote6
Adventurer
Have the fortress (or at least the inner keep) be carved from a poisonous abyssal stone. The very substance of the rock is toxic, and acts like some poison. Pick your favorite.
Me, I'd go with something that does relatively little damage (say, 1d2-1d4, or even 1d3-1), possibly not so tough* (say, Fort save DC 14 -- not so hard for 20th level characters), affects either Wis or Cha (or both, randomly -- roll each time the character takes damage), and maybe has some kind of other effect (e.g., madness of some sort, and/or loss of self-volition when the stat reach 0).
The thing is, the PCs will be continuously exposed. Every minute they're there will call for a save (or two -- one for this minute's exposure, one for last minute's). Those who aren't out-and-out immune will slowly succumb to madness, losing their will and/or their self-image. (At two saves a minute, failing one in 20, taking 1 pt of damage per failure, an hour of exposure will cost 6 pts of ability damage. Ouch.)
If they aren't careful, the reach the demon queen just in time to throw themselves at her feet, and worship her as their Dread Mistress.
Otherwise, they'll probably be burning through neutralize poison, heal, and the various restoration spells like crazy, which will chew up resources & spell slots.
*This assumes you use the rule that a 1 on a save is automatically a failure; if not, beef up the save DC.
Me, I'd go with something that does relatively little damage (say, 1d2-1d4, or even 1d3-1), possibly not so tough* (say, Fort save DC 14 -- not so hard for 20th level characters), affects either Wis or Cha (or both, randomly -- roll each time the character takes damage), and maybe has some kind of other effect (e.g., madness of some sort, and/or loss of self-volition when the stat
The thing is, the PCs will be continuously exposed. Every minute they're there will call for a save (or two -- one for this minute's exposure, one for last minute's). Those who aren't out-and-out immune will slowly succumb to madness, losing their will and/or their self-image. (At two saves a minute, failing one in 20, taking 1 pt of damage per failure, an hour of exposure will cost 6 pts of ability damage. Ouch.)
If they aren't careful, the reach the demon queen just in time to throw themselves at her feet, and worship her as their Dread Mistress.
Otherwise, they'll probably be burning through neutralize poison, heal, and the various restoration spells like crazy, which will chew up resources & spell slots.
*This assumes you use the rule that a 1 on a save is automatically a failure; if not, beef up the save DC.