Soul Cairn (Dawnguard)

Soul Cairn is a plane of Oblivion and an ethereal realm of trapped souls where the undead roam. It is accessed via a mysterious portal in Castle Volkihar during the quest Chasing Echoes.

Geography
The staircase that the player descends to enter the Soul Cairn is located in the south-western end of the map. The prison holding Valerica is directly north-east of that staircase (with the path connecting the two running SW to NE), and roughly halfway between them is a large, fortified wall cutting across that path near-perpendicularly: NW to SE. The point where the two intersect can be useful as a tool in navigation and orientation around the map.

Soul Fissures
There are Interactive cracks in the ground and by touching them will randomly fill a soul gem in your inventory.

Reaper's Lair
It is located in the northern south-east edge of the Soul Cairn, past the stone wall. Inside lies the Reaper Shard Receptacle, which is activating by placing 3 reaper gem fragments in the receptacle. These fragments are located in chests beneath 3 giant floating soul gems which will sap your health when you get close. Lightning frequently strikes the centre of the room. Putting enough shards on the receptacle summons The Reaper.

Boneyard
Accessed after killing three Keepers. Valerica is trapped here together with one of the three Elder Scrolls. After the Dragonborn enters Boneyard, Durnehviir will attack.

Lightning Attractor
There are at least three Lightning Rod locations in the Soul Cairn. 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. Activate these by using Grand Soul Gems or Greater Soul Gems. Lightning will strike the Rod, and various enemies (Such as Mistman, Boneman, and Wrathman) will appear and the soul gem will be replaced by a Black Soul Gem. When striked, the rod will slide into the orifice and the Lightning Attractor will not be activated a second time.

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 Boneyard Keepers' demise. Once you defeated all of the Boneyard Keepers the wall that blocks you off from the castle dissappears. Once you enter Durnehviir will appear and attempt to kill the Dragonborn. Once you defeated Durnehviir, you will speak with Valerica and then will lead you to the Elder Scroll. As soon as you exit you will see Durnehviir outside and he will talk to you then he will teach you Summon Durnehviir.

Impatience of a Saint
The Dragonborn can interact with the spiritual essence of Jiub and accept a quest to find the second volume of his autobiography, its pages are scattered across the barren landscape of Soul Cairn. There are 10 pages in total to find.

Find Arvak's skull
In addition, the Dragonborn can complete a side quest to free an undead skeletal horse named Arvak. This mount can then be summoned at will in either Nirn or Soul Cairn. Arvak's Skull can be found under a pavillion that is glowing on the ground near the big castle. Returning the Skull to the owner will reward the Dragonborn with the Summon Arvak spell.

Bring Morven Stroud 25 Soul Husks
There is a soul, named Morven Stroud, directly east when you pass the wall. Speak to him and he will ask you to bring him 25 soul husks. If you bring him these items he will give you the choice between a few items to trade for the 25 husks. This quest is repeatable and the weapons come with enchantments. Be sure to look in the area around him for the obelisks with glowing orbs, hit them directly with a spell and the bars of the doorway to the building will open.

A glowing pit portal to the right of Morven Stroud will teleport you to on top of a building. If you look to the building with the obelisks with glowing orbs, there is a spot you can jump down and there will be a hidden chest to be found.

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.
 * You cannot enter the Soul Cairn without Vampire Lord affliction or being partially soul trapped.

Gallery
Soul Cairn (Dawnguard)