Soul Cairn (Dawnguard)

It is a plane of Oblivion inhabited by trapped souls and undead. It can be accessed via a portal in Castle Volkihar during the quest Chasing Echoes.

Background
When a soul is trapped in a soul gem and its energy used to power an enchantment, the remnant - the soul itself - is sent to the Soul Cairn. It is a place devoid of life - the only beings that exist there are souls, undead and the rulers of the realm, the Ideal Masters.

The Ideal Masters are enigmatic entities that lord over the Soul Cairn, controlling every aspect of its being. It is believed that the Soul Cairn exists so that the Ideal Masters can collect souls and feed on them; some theories even suggest they use the souls as a form of currency to an even higher power.

The Soul Cairn is also home to powerful undead beings, which are highly sought after by necromancers. Souls are sent to the Soul Cairn by necromancers who wish to summon these undead, but in order to do so they must travel to the realm in person to finalize the deal with the Ideal Masters. They are often tricked, and many become trapped as harvested souls.

Geography
The landscape of the Soul Cairn is barren and dry, with only dead trees and bushes scattered throughout the realm, as well as plant-like growths known as Soul Husks.

The sky is perpetually bleak and cloudy, and lightning strikes constantly throughout the realm. There is no sun in the sky, but instead a huge black, circular void.

Numerous buildings and altars can be found throughout the land, alongside mysterious upright stones and circular crystalline structures. The crystal circles will replenish the health of any being that stands in them, although they will quickly dim and become inert after use. In the tallest towers, the uppermost sections are composed of blocks suspended in midair, rotating around the tower and emitting a purple light.

The realm is partitioned by an enormous fortified wall that divides the landscape. Magical barriers also block off certain areas.

Soul Fissures
There are luminescent cracks in the ground throughout the Cairn. Interacting with one will fill the largest empty soul gem on hand.

Crystalline Structures
Scattered across the Cairn are a number of floating purple crystals that resemble giant soul gems. Approaching these will drain the health of any living being that gets too close. Eating a Soul Husk, however, will counter the drain for a short time.

Lightning Attractor
There are four Lightning Attractors located in the Soul Cairn. Activate these by using Grand Soul Gems or Greater Soul Gems. Lightning will strike the rod, and Mistmen, Bonemen, and Wrathmen will appear. Also, the soul gem will be replaced by a Black Soul Gem. After being struck, the rod will slide into the orifice and the Lightning Attractor cannot be activated a second time.


 * One is located by one of the Keepers, to the left of where the Portal is.
 * Another is located near Arvak's Skull by the glowing castle.

Reaper's Lair
While inside the Soul Cairn, if you turn right after going through the stone wall and follow the barrier along, just past one of the large towers with floating blocks and swirling energies at the top, you will come to the Reaper's Lair. Inside lies the Reaper Shard Receptacle, which is activated by placing three reaper gem fragments in the receptacle. These fragments are located in chests beneath three of the giant floating soul gems which drain health. Lightning frequently strikes the center of the room. Putting three shards in the receptacle summons The Reaper.

Boneyard
The Boneyard is accessed after killing the three Keepers. Valerica is trapped here along with one of the three Elder Scrolls. Shortly after the Dragonborn enters the Boneyard, Durnehviir will attack.

Chasing Echoes
Serana's mother, Valerica, is found here. She offers an Elder Scroll to the Dragonborn if the Boneyard Keepers located at the tallest spires in the Soul Cairn are defeated. She warns that a fearsome Dragon named Durnehviir may take notice of the Keepers' demise. Once all of the Keepers have been killed, the wall that blocks access to the castle disappears. After entering the Boneyard, Durnehviir will appear and attempt to kill the Dragonborn. Defeat him and speak with Valerica, who will lead the player to the Elder Scroll. After acquiring the Elder Scroll and exiting the Boneyard, the Dragonborn will find Durnehviir waiting on a broken structure. After conversing with him, he teaches the Dragonborn the a shout to call him from the Soul Cairn.

Impatience of a Saint
The Dragonborn can interact with the soul of Jiub in the northeast section of the Soul Cairn. Jiub explains that he wants to write the second volume of his opus, but he has lost the pages to his first volume and requires them to continue writing. There are ten pages in total, scattered across the barren landscape of the Soul Cairn.

Find Arvak's Skull
The Dragonborn can also complete a side quest to free an undead skeletal horse named Arvak. This mount can then be summoned at will in either Nirn or the Soul Cairn. Arvak's Skull can be found under a glowing pavilion near the big castle. Returning the Skull to its owner will reward the Dragonborn with the Summon Arvak spell.

Soul Husk hunt
There is a soul, named Morven Stroud, in the northern section, directly east of the dividing wall. Speak to him and he will ask for 25 soul husks in return for the opportunity to 'see his wares'. For every 25 soul husks the Dragonborn brings to him, they receive the opportunity to trade for a random sword, battleaxe, light armor, heavy armor, or spell tome. This quest is repeatable and some of the items he has are enchanted.

A glowing pit portal to the right of Morven Stroud will teleport one to the top of a building. Look to the building with the obelisks with glowing orbs, there is a spot to jump down to get a hidden chest.

Trivia

 * While in Soul Cairn, a soul might say, "Death is but a door and time is but a window; I'll be back", which is a reference to Vigo the Carpathian from Ghostbusters II.
 * One cannot enter the Soul Cairn without being a vampire or by being partially soul trapped.
 * A soul may talk about an Imperial Battlemage, which is a reference to, where you can go to the Soul Cairn.