How do I...?

[Squire James]

Of course, if you're going to go that far, you might as well make that rat a wizard's familiar delivering a fear effect... they don't even need to see the wizard this session. In fact, you can foster a great hatred within the PC's of a guy they've never seen, simply from an occasional "rat delivery" at various points of the campaign!

 

[(Psi)SeveredHead]

Of course, if you're going to go that far, you might as well make that rat a wizard's familiar delivering a fear effect... they don't even need to see the wizard this session. In fact, you can foster a great hatred within the PC's of a guy they've never seen, simply from an occasional "rat delivery" at various points of the campaign!

That is a wonderfully evil idea Maybe you can start using illusory rats once the PCs figure out what's going on. Each illusion looks exactly like the (distinctive) rat in question, and can cause PCs to waste spells trying to "kill" it.

 

[Nellisir]

Going back to the original idea...you don't want to simply inflict something on the characters. You want the penalty (a twisted ankle) to be a result of actions the characters (and players) willingly take.

Traversing a long passage is normally something that can happen "offscreen", so to speak. You don't need to be in combat time. Characters can be assumed to be taking 10.

So the trick is, set up an environment that the characters can easily traverse while relaxed and moving slowly, but that gets treacherous at faster speeds. DMG pg 60 covers floors. An uneven flagstone floor requires a DC 10 Balance check to run or charge across; failure means the character can't move this round.

So, make the floor uneven flagstone (or whatever, but with the same qualities), replace the "can't move" penalty with a -2 to Dex, -10ft reduction in speed for 4d6 rounds (or minutes or hours), and then just get the PCs running (scare them, a fear effect, something).

It's essentially just a trap.