Apocrypha

Apocrypha is a Plane of Oblivion created and ruled by Hermaeus Mora, the Daedric Prince of Fate and Knowledge. It is described as an endless library consisting of books with black covers and no title, where all forbidden knowledge can be found. The plane is covered in acidic liquid which can kill anyone that enters it, as well as housing deadly tentacles. Lurkers, a form of servant for Mora, live there, waiting for the next traveler to ambush. Seekers guard the forbidden knowledge here, attacking foes with powerful sound attacks.

Mortals can enter the realm through reading one of the Black Books, which are among Hermaeus Mora's artifacts. The Black Books are tomes of forbidden knowledge displaced from time and scattered across Tamriel. These Black Books are not portals to Apocrypha, however. They transport the reader's mind to Apocrypha but his physical appearance stays on Tamriel. Most mortals that read these books are driven mad by unending revelations. Such was the fate of the mage Morian Zenas, who disappeared in Apocrypha during his voyage through Oblivion. Those that have successfully journeyed through Apocrypha have discovered powerful knowledge.

During the Merethic Era, the Dragon Priest Miraak came into possession of one of these Black Books which led him under the servitude of Hermaeus Mora. He organized a rebellion against his dragon overlords. When he initiated the rebellion, he used the power of the Voice and his own power as the Dovahkiin to destroy his dragon overlords. However, he wasn't powerful enough to do so. Even though he slew many, in the end, the dragons razed his temple and destroyed his servants. Skaal legends speak of another dragon priest, Vahlok, who learned of his plans and confronted him, as the Dragons had sent him to defeat Miraak. The battle lasted for days and was of such large scale that the power released from it separated a piece of Skyrim's land, which became the island of Solstheim. Vahlok eventually managed to defeat Miraak and, when he was about to end his life, Hermaeus Mora intervened and transported Miraak to Apocrypha, saving his life. Vahlok was then made his jailer and was tasked to wait for his return.

In the Fourth Era, Miraak attempted to harness the power of the All-Maker Stones to get enough power to return to Solstheim. The Last Dragonborn learned of his plans, gained the power that Miraak possessed and confronted him. The two fought and as the Last Dragonborn was about to deliver the final blow, Hermaeus Mora intervened again, this time not to save Miraak, but to kill him. Subsequently, Mora accepted the Last Dragonborn as his servant.