Black Book: The Sallow Regent (quest)

Walkthrough
After defeating the dragon priest Dukaan, the Dragonborn finds an altar which holds the book.

Tip: Using the Candlelight spell is recommended.

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

Survive the trip through Apocrypha
The Dragonborn must then defeat the lurkers and seekers along the way until he/she reaches 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 White Ridge Sanctum.

de:Schwarzes Buch: Der bleiche Regent (Aufgabe)