Finally, all your hard work and time will result in two things on this last level.
The first will be learning who took the Heartstone Gem and being able to slay this foul person.
The second will getting the Heartstone Gem back from their clutches, but doing so will not be an easy task.
I cannot stress enough to you that magical weapons of 2+ or greater are required to bring the foul Yxunomei down who is in fact using the body of a child to hide her true form but don't fret brave warrior, you'll get to see her soon enough. Your journey through the twisting passage, which as you can see from the map is in the form of a snake, will lead you to the chamber that Yxunomei rests and going into any of the rooms off to the side from the main corridor is not needed, although for experience and the items found within, it's worth your while.
Step carefully while moving through the snake like passage for there are pressure points of poison dart traps and paralyze traps that will cause your party grief, especially if they are attacked by the very skilled and excellent Yuan-ti Archers whose arrows seem to always strike true. A paralyzed character while being pelted by three Yuan-ti Archers and their arrows does not stand a chance and I suggest you engage sword to sword combat as soon as possible so they change their weapons to something more appropriate that will stop them from launching their arrows at you. Do pick up their arrows if you have an archer in your party, they will help you in bringing down Yxunomei once you come face to face with her.
The final double doors are guarded by Yuan-ti Archers and Yuan-ti Priests as well as Yuan-ti Elite guards and the very back line of Yuan-ti Archers are standing on pressure traps, so do your best to avoid stepping on them. I found a few fireballs in their general direction did the trick. Once they are taken care of, you want to memorize some defensive spells and good offensive spells for the battle against Yxunomei. This is what I found to be very useful during this battle and I did not have anyone drop from her attacks, although it was my fourth time attempting it.
Weapon wise, I found the majority of my +2 weapons in Dragon's Eye and equipped my characters accordingly to give them the best chance of victory.
My strategy was simple as I created a front line of my 4 strongest characters and had the middle one walk into Yxunomei's Chamber and lure her out into the hallway, where I brought that character back to the front line so I could get all 4 characters attacking her. My thief, who was now using the arrows from the Yuan-ti Archers was behind this line away from any attacks by Yxunomei and my mage was off to the side so when Yxunomei attacked, the Agannazar's Scorcher I'd cast would not damage or hit any of my front line. Use the space bar when you have to to monitor your party members and health status. Having Bless and Recitation gave my characters a +3 to hit and +4 to Saving Throws and gave Yxunomei a -3 to hit and -4 to her own Saving Throws. This defensive spell tactic helped greatly and I dropped Yxunomei very quickly without any loss of lives within my party.
Readers sent me a few notes on a secret room in the bottom right corner at the back of the torture chamber. Search the area and you will gain access into another room where you will find a few Yuan-Ti Champions. Kill them, search the room and get the Throwing Axe +2 which comes back to you when thrown!
Here is some more information on the secret room from another reader: "Dragon's Eye, Level 5. In the torture chamber SE of the path next to the bookshelf there is a stone which shows blue. Press that, you hear a click, head back towards the path and there's a new side passage leading to the torturer and a couple of his henchmen. Dispatch them and on the table you find a +2 axe of returning (no Dwarf should be without one) and a couple of decent scrolls."
Thanks to Peter and Errand for this hidden gem!
Pick up the Heartstone Gem, clear out the rest of the level as you see fit and return to Kuldahar to speak with Arundel.