A trap door opens and one random character slides down to a spinning circular saw. A roll to Hit applies, or perhaps a Dex roll, or Saving throw (whatever the GM prefers). If hit, the character takes 5d6 HP of damage, and the character then lands in a locked room on the floor below. Optional: If struck, in addition to the listed damage, roll d100: 01-50% means something has been cut off; if so, roll d100 again: 01-20% left leg, 21-40% right leg, 41-60% left arm; 61-80% right arm; 81-00% head (instant death).
Circular saw 3 by < href="https://openclipart.org/user-detail/Firkin target="_blank">Firkin. Modified by David in Inkscape. Creative Commons Zero 1.0 Public Domain License.