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Shoot the rope bow at the ceiling beams to climb up.
This will take you to a secret area with a Poison Bow and some potions in a chest. Defend her from the Orcs inside (use the spikes and fire), and you'll have averted this brief crisis. Leanna is a little gung-ho, so don't try for stealth here.
There's a Ring of Regeneration in here, and some arrows and potions, too.
Xana makes the solution to this puzzle all too clear; go up to the wall by the fire and shoot the Rope Bow up at the wooden beam.
When you've eliminated them go through the passage at the top of the sand hill.
Find a way out of the Spider's Lair: This place is completely covered with web and full of much nastier spiders than the ones you've encountered so far.
Climb up and look for the lever to the right to open the way.
Find the Crystal Chamber and restore the temple's energy: Just up ahead is a room with a few Orcs mulling about and a rather impressive-looking gate. Leanna will come in after it's nice and safe, and move a large obstruction out of the way so you can flip a switch and open the portcullis.
She'll be waiting for you right where you left her.
Kill the Pao Kai: After Leanna briefs you, that dragon will bust in and start raising hell again. Run back through the portcullis where you last came in, wait for the Pao Kai to follow, and drop the gate to run him through.