Mechanical trap
Along the side of the wall is a very obvious slit in the stonework of the wall. A pressure plate can be spotted in the middle of the floor with a DC 10 Wisdom (Perception) check. Upon inspection, characters will see an iron axe blade inside the slit with a chain leading further back into the wall.
The pressure plate is fake and so is the the blade. If anything is placed inside the slit, or if the pressure plate is manipulated, a 10’x10′ section of the opens further behind the trap (to catch party members other than the one examining the trap ).
The pit is 10′ deep with iron spikes. A creature falling into the pit takes 1d6 falling damage plus 2d10 piercing damage from the spikes. The real pit trap can be detected if looking behind the false trap on a DC 20 Wisdom (Perception) check.