Hail the Keeper of the Bridge. He tells you "Only the willing touch of fire will extend the bridge. Each flame has its place. You must sacrifice your flesh to find that place. Once the bridge is extended, you must cross it to prove your faith."
There are 9 numbered torches placed in a random order around the lava pool, with a ramp down to the lava. 5 torches to the left and 4 torches to the right of the Keeper of the Bridge. Each torch activates a portion of the fire-bridge you must cross to get the quest update.
In numerical order, right click on each torch and place your hand in the flame to extend the bridge. Keep in mind the following
You'll have to place you hand in the flame of each torch at least once and make a note of its number, then go back around and light them in the proper order.
If you light any torch out of order, any torches you have lit in the proper order will flicker out, and you must start over with Torch
Once all torches are lit in order, walk across the bridge. Despite what the Keeper's words implied, it seems to work either direction. The quest will update about halfway across.
Click on the door you entered by to zone out of the trial instance.