Book of Fel Names

A tome said to contain the true name of every demon in existence, as well as priceless information about the nature of demons, their weaknesses, and their habits. Varedis Felsoul once absorbed the power in the book, but he did not steal it and was only linked to it. When adventurers destroyed the book, Varedis was weakened.

Source
This quest item drops from Blackheart the Inciter when players are on the quest The Book of Fel Names.

The book is returned to you when you start the next quest called Varedis Must Be Stopped.

As a quest objective

 * The Book of Fel Names
 * Varedis Must Be Stopped