The carpet on the floor of the room is a projection,
it is a 20' deep pool of water.
Anyone attempting to walk into the room gets a passive spot or search chech.
Fails of the DC25 Passive Spot or DC16 Search will result in them walking into the pool.
At that point, one last RefDC20 in the featureless hallway can prevent dropping onto the pool.
Once one or more folks drop into the pool, a trapdoor in the ceiling dumps blocks of ice into the pool,
AoE, 3x SwimSaveDC20 1d4
and the impeller at the bottom starts churning.
To exit, the victim must make a Swim DC20 to get to the outside of the rotating icy vortex,
followed by a RefDC20 or StrDC15 to pull himself out (1max dry AideOther +2) .
Each failure moves the trapped closer to the bladed impeller,
from the Ledge to the rotating ice, to the vortex,
then its a permanent vacation in Margaritaville
<or, if you want, SwimDC75, 100d6, back to 'Rotating Ice' and repeat>