Black Book: The Hidden Twilight (quest)

Walkthrough
After completing the Reluctant Steward quest, the Dragonborn received a key to a previously locked door which permits access to an altar which holds the book.

Read the book
The Dragonborn reads the book which transports to the realm of Apocrypha.

Survive the trip through Apocrypha
The Dragonborn must then defeat the lurkers and seekers along the way until reaching another black book. Upon reading the book, three balls of green light appear, each corresponding to a new ability. The Dragonborn then chooses one and is transported back to Tel Mithryn.

de:Schwarzes Buch: Das verborgene Zwielicht (Aufgabe)