Travel: Valley of the Dead
Now that you've found the Sisters - and survived their treachery - it's time to head to the Valley of the Dead. Fortunately, in tracking the Sisters down you also claimed the Cyclops Incense Burners they held, which are the keys to gaining access to what they sought. If anything could bring ruin to a city like Varnhold, a place named the "Valley of the Dead" seems to be an appropriate place to search. To get there from the Kellid Barbarian Camp, follow these directions:
Along the way there, you'll be stopped by a not-so-random encounter, where your old friend, the black bird, will show up and bother you again. This time it has a generous offer for you: submit to its master, Vordakai, the Chosen of the Four Horsemen and the Keeper of the Oculus of Abaddon. After empty title-reading, the bird will admit its master was indeed behind the Varnhold vanishing, and will instruct you that you need three incense burners - which you should already have.
After that encounter, you can continue on your way:
Opening the Gate
When you arrive, follow a trail north a short distance to reach a gate. Interact with it and pick the option [Light the three incense burners from the tombs and hang them on the hooks on the center of the gate] to open the gate in... somewhat dramatic fashion. Continue north past the gate to find an area transition to Vordakai's Tomb.
Warning!: Before you enter Vordakai's Tomb, make sure you have plenty of rations - enough to rest several, perhaps half a dozen times. It's heavy, but once you enter, you're not leaving, and there are hordes of nuisance monsters in the dungeon ahead. Energy Drain and Ability Damage will be your constant companions, so spells like Delay Poison, Lesser Restoration and Death Ward are necessary and may require constant application. You have been warned.
Warning!: If Tristian is your primary divine spellcaster and you're dead-set on bringing him with you, you may want to adjust your party to include a secondary divine spellcaster... or at least a character with access to Remove Paralysis. Jaethal is an ideal candidate.