Search for the Archives of Zoltun Kulle in the Desolate Sands
This quest will begin automatically after you've finished the previous one. After you've returned to the Desolate Sands you will have to start searching for a passageway leading to the last portal. It's best to explore the northern part of the location while moving close to the edges of the map.
Zoltun Kulle will activate the last portal and you must use it to travel to the Archives of Zoltun Kulle.
Go to the Terminus
Start running forward and make a stop once you've located a new waypoint. You should also encounter Emperor Hakan II and Leah here. Continue your journey until you've reached the Terminus.
Unknown Depths Shadow Lock opened
Listen to a new conversation and then approach a portal which will transport you to the Unknown Depths. This is the first location where you'll have to use the mage's blood to activate a lock.
You must be very careful, because it's very dangerous here. Watch out mostly for Rock Giants, however you'll also have to acknowledge the presence of the Fallen and their shamans, Guardian Towers and various types of skeletons and spiders. It's also crucial to quickly destroy all Toxic Constructs before they summon too many monsters.
The depths are a large location and your objective is to reach the opposite end of the map where you'll have to open a Shadow Lock. The first part of the main mission is now completed. Find a portal nearby to go back to the Terminus.
Storm Halls Shadow Lock opened
You must now travel to the second portal which will transport you to the Storm Halls. This location is similar to the depths, so it's big and it's populated by large groups of powerful monsters.
Continue exploring the halls until you find the second Shadow Lock. Open it and then use a new portal to go back to the Terminus.
Enter the Realm of Shadow through the Terminus
Proceed to the third portal which will teleport you to the Realm of Shadow.
Find Zoltun Kulle's body
This is a dark location occupied by many tough monsters. Shade Stalkers are little demons and they're hard to track and Sand Behemoths are extremely resistant to attacks. Aside from them you'll also encounter plenty of wraiths, spiders and Guardian Towers. Approach Zoltun Kulle's body with caution, because it will be heavily defended.
Talk to Leah in the Terminus
Once you've collected the body use the portal to return to the Terminus. Talk to Leah here and she'll use her powers to resurrect Zoltun Kulle.
Enter the Soulstone Chamber through the Terminus
Zoltun Kulle will open a portal to the Soulstone Chamber and you must follow him there.
Talk to Zoltun Kulle
Head forward and get ready for an unpleasant surprise - a new battle.
Kill Zoltun Kulle
The best way to begin this battle is to eliminate the Eternal Guardians. They are quite tough, however they're also very slow. Try to avoid Zoltun's attacks in the meantime, because he won't wait until you focus on him.
Zoltun Kulle himself has an ENORMOUS life bar. He likes to cast a spell which slows down your team, assuming of course you will find yourself within the radius of the spell. Other Kulle's attacks are a whirlwind which inflicts major injuries and fireballs which explode after coming in contact with a target. Kulle can teleport himself across the battlefield which makes killing him even harder. Expect this duel to take a large amount of time and wisely use your health potions and skills.
Get the Black Soulstone
Once you've eliminated Kulle and his servants you will be allowed to collect the Black Soulstone which is in the middle of the chamber. You must now take part in new conversations with Leah and Tyrael. As for the soulstone, Adria will take it.
Talk to Adria in the Hidden Camp
Use a portal to travel back to the Terminus, because you can't use the town portal spell within the Soulstone Chamber. Return to the Hidden Camp and talk to Adria again to end the quest.