Soul Cairn (Dawnguard)

It 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.

Interactions
It is accessible to the Dragonborn after completing the quest, 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.

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 the Soul Cairn.

Trivia

 * If the player looks at their map while in Soul Cairn, it will say that they are in the Sea of Ghosts.
 * 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 Ghostbusters.