(I'm assuming that Connor threw the stone with light on it before Feng tried to jump across the hallway, creating illumination down the hall, enough for those with darkvision to see the arch....)
Feng makes a running leap down the hallway, he falls quite short, and lands hard on the ground, grunting in pain. As he starts to get up, there's a quiet, but very audible click sound, and the stones beneath him fall away.
Even his uncanny barbarian reflexes aren't fast enough for Feng to roll away, and he falls down a deep pit. Feng takes 15 points of bludegeoning damage from the fall. At the bottom of the pit are several large, sharp spikes. One of the spikes hits Feng's breastplate hard enough to leave a dent in it, but Feng falls off to the side of the spike, unharmed.
The rest of the party can hear a loud ringing sound from up ahead. The pit is nearly one-hundered feet across, with only a very small ledge around the sides of the hallway.