Forbidden Knowledge

Acquisition
Forbidden Knowledge is a quest acquired from a letter in your Writing Desk in the Hawke Estate. If not killed during Enemies Among Us, Idunna sends you a letter:

"Serah Hawke, Years past, I fell into wickedness and was swept up in the mad ambitions of the blood mage Tarohne. You spared my life at the Blooming Rose, and for many months, I wished you had killed me. But I found the Maker's Light in my deepest despair, and I now seek absolution.

Tarohne wrote many tomes containing dark secrets she learned from the Fade. She protected them with demons forged into their very bindings. You must find and destroy these tomes, lest others follow her dark path. I wish I could help you, but the templars never let me leave my cell. I can only send my prayers.

-Idunna."

This quest does seem to be available if you killed Idunna, by simply finding one of the tomes and triggering the sequence. If you kill Idunna you do not receive the letter but you can still do the quest. If you destroy the first tome, you obtain the quest. The quest was obtainable after reading the first tome come across, but it wasn't possible to complete the quest. The quest is obtainable even if you read the first tome, but it is highly discouraged to read any tome but the last.

Walkthrough
After entering an area with a tome, the map will show an arrow towards it. Be prepared to fight several packs of demons in each cave. This quest should be completed in the second act.


 * A cave at the west end of Sundermount, next to the western exit. Newly opened passage after the beginning of act 2. Cave is known as the "Abandoned Thaig" and has a nexus golem with wares in the middle chamber with the tome.
 * A cave at the east end of The Bone Pit (outside the mine). Inside the location is called "Rotting Cave"
 * A cave in the northern middle parts of The Wounded Coast. It should be named "The Dank Cave" and is not marked on your map. Note that this cave is inaccessible later in the game.
 * In The Chantry, on a table from the southern stairs, second floor.
 * In the Viscount's Keep, eastern part on a table near the throne.

You can choose any of the following options when selecting a book (Friend/Rival points for first book only): Choosing any of these options will cause several demons and abominations to appear and attack.
 * (Destroy the book.) 
 * (Read the book.)
 * (Accept the book's deal.)  Item receieved: Greater Tome of the Mortal Vessel +2 attribute points for Hawke
 * (Take the book.) 
 * (Take the book.) Item received: Book of Forbidden Lore

NOTE: Reading and accepting the book's deal will give you +2 attribute points, but this will only happen once, any books read after this will not give any attribute bonuses. Also, taking or reading any of the books makes the player unable to complete the quest, and the quest will remain in your journal. However, if you ONLY read the last book (named The Fell Grimoire) you will complete the quest, fight Xebenbeck and get 2+ attribute points.

After destroying the five Evil Tomes, your Journal will update, and you will get information on The Fell Grimoire. It can be found in The Forgotten Lair, which is a cave in Darktown next to the entrance. Note that you may come across a locked door and will need a Rogue to unlock it, in the Forgotten Lair. Inside the cave there are several traps. Some of them are capable of killing most members of your party rather quickly so watch out for them, there's spaces where you can walk without triggering them.

After destroying the Grimoire you will meet one of the most dangerous bosses in the game, Xebenkeck, a powerful Desire Demon. The boss itself is not very hard but she will summon hordes of Rage Demon's and Abomination, as well as a Revenant in the second wave.

NOTE: If you choose to take the books, you can sell them each 86 pieces. 5 books= 4 sovereigns 38 pieces.

Result
The Fell Grimoire and Tarohne's tomes were destroyed.

Rewards
Voracity can be found inside the body of Xebenkeck and you are rewarded with the achievement "Exorcist" after killing her.