[As they're about to turn the corner, she stops and points to a decomposed wooden part of the otherwise stone ceiling. Under it is a pile of rocks high enough to nearly fill the other passage, blocking it out.]
That one was there before. It's there to nab whoever sets it off, but it's ancient, so the wood rotted and the trap set itself off instead.
no subject
[As they're about to turn the corner, she stops and points to a decomposed wooden part of the otherwise stone ceiling. Under it is a pile of rocks high enough to nearly fill the other passage, blocking it out.]
That one was there before. It's there to nab whoever sets it off, but it's ancient, so the wood rotted and the trap set itself off instead.