Resurrection Gorge Overview - Baldur's Gate 2
Resurrection Gorge is the final map that you'll be visiting during Dorn's Companion Quest. This map can only be unlocked by progressing Dorn's quest until the very end, after clearing the Helmite Camp during his quest you'll get waylaid by another group of Paladins who want revenge. After defeating them you'll be directed to this gorge and it'll be marked on your map.
The upper portion of Resurrection Gorge is as barren as you can imagine. You'll find a few very low level animal enemies in the area that aren't even worth the time it takes to hunt them down.
Summerheigh: When you first run by her she'll stop you and give you some background information about the area we're in. Aside from that she is a vendor who sells a variety of different normal weapons. She will also identify any items you have too.
Treadsoft & Yarrow: Your first encounter with Treadsoft here will result in a fight; when his health gets low enough Yarrow will teleport in and talk to you. If you decide to kill Treadsoft and Yarrow then you'll want to pick up the Acorns of Yarrow from the ground and put them into your quick slot. You'll have to use this item to teleport to and from the top and bottom of the gorge. Alternatively, if you let Treadsoft and Yarrow live then you'll want to speak to Yarrow for transportation into the gorge.
The bottom portion of Resurrection Gorge can be reached by speaking with Yarrow and/or killing Yarrow and looting the Acorn of Yarrow. Even if you're playing a good character I would recommend killing Yarrow (there is no Reputation loss or penalty for doing so) as it'll make navigating this zone much easier. Having the Acorns of Yarrow on you mean that you can enter/exit the gorge from anywhere at any time.
In the bottom portion of the gorge your task is to collect the 3 missing Summoning Stones and then perform the ritual with Dorn. To perform the ritual you'll need to sacrifice either a summoned creature, a companion or Vernus who is already in the Summoning Circle. One thing to note too is good aligned characters like Anomen and Keldorn will object to this ritual and leave your party if you try to perform it. Make sure they don't have items you want in their inventories (like the Summoning Stones) or you're going to be really inconvenienced.
Summoning Circle: Inside this tree is where the Summoning Ritual with Dorn will take place. For more information about this part of the quest check out Dorn's Companion Quest walkthrough.
Winterbrook: One of the Summoning Stones you need for the ritual can be found in a cave behind Winterbrook. It's hard to see but if you mouse around this area you'll see that you can enter into a cave at the end of the path behind Winterbrook.
Magnificent Fil: Magnificent Fil has one of the Summoning Stones we need to complete the Summoning Circle. You can trade a Purple Crystal to Magnificent Fil for it, pickpocket/steal it from him or kill him - the typical 3 D&D options. Purple Crystals can be found north of Fil, in the area with all the Corrupted Ankhegs.
Potion: At this location you will find a Potion of Clairvoyance which will allow you to see invisible creatures for 10 minutes. Use this potion or Detect Invisibility, Glitterdust, Invisibility Purge or True Seeing to reveal Xachrimos in the lower gorge. Until you use some method of detect invisibility all you will see is Xachrimos' speech bubbles as he roams around the area.