If Idunna was not killed during Enemies Among Us, she sends a letter to Hawke at the beginning of Act 2. If the blood mage is dead, the quest is received upon finding any Evil Tome.
The order in which you find the Evil Tomes is irrelevant. Upon entering an area with a tome, it will be marked on the map. Be prepared to fight shades, demons and abominations. The tomes are located:
In the chantry on a table near the top of the southern stairs.
Note: When the enemies spawn a door closes. It can be unlocked (during combat only) for 250 XP with 50 cunning (or 40 and the Caster's Skeleton Gloves).
Note: The tome in the keep can not be used immediately upon arriving in Act 2; Hawke must leave the area and return to use it. (This may not be a bug as Hawke being able to use the book immediately upon starting Act 2 would mean they could possibly be forced to fight alone if they did not use the horn in the keep to gather their allies prior to reading it.)
In the Rotting Cave, whose entrance is found at the far-eastern end of The Bone Pit at surface-level, at an otherwise dead end (see map below). A Corrupted Lyrium is also in the cave.
In the Abandoned Thaig (aka Hidden Dungeon during Act 3), whose entrance is shown as "Recently Opened Passage" in the game, and is very near Sundermount's western exit to the world map. The tome is found in the main room of the thaig, along with the Nexus Golem's wares. There is a battle in this room before getting the tome; see abandoned thaig strategy if you have trouble.
In a Dank Cave, whose (unmarked) entrance is found in the north-central area of The Wounded Coast (see map below). A dragon guards the tome and a modest treasure. There is also Raw Lyrium at the beginning of the cave.
You can choose any of the following options when selecting a book (they are all mutually exclusive):
Destroy the book:
Merrill: rivalry (+10) (if Merrill is already romanced or has already reached +100 friendship, she won't disapprove)
750 XP (1500 XP in pc1.04)
Read the book and accept the book's deal.
Merrill: friendship (+5) (This will only apply for the first book.)
Greater Tome of the Mortal VesselGreater Tome of the Mortal VesselTome This tome gives the reader one or more points to spend on attributes, but is rendered useless afterwards.
+2 attribute points
Take the book.
Merrill: friendship (+5) (This will only apply for the first book.)
Choosing any of these options will cause several demons and abominations to appear and attack.
Reading and accepting the books' deal will give you +2 attribute points, but this will only happen once: any books read after this will not provide attribute bonuses. Taking or reading any of the Evil Tomes will make the quest impossible to complete, and it will remain in your journal. However, if you destroy all five Evil Tomes, the quest will proceed and you will be led to the last book called the Fell Grimoire.
The Fell Grimoire can be found in The Forgotten Lair, which is an area in Darktown next to the entrance. You may come across a locked door (10 cunning, 50 XP) and will need a rogue to unlock it in the Forgotten Lair. Inside the cave, there are several complex (30 cunning, 150 XP) and master level traps (40 cunning, 200 XP) that can be inconvenient when also being attacked. Ranged attacks can draw enemies into safe areas where they can be defeated and then the traps can be disarmed.
Bug! A rogue Hawke or companions may not be able to disarm the three master level traps with 40 cunning. Someone Confirmed: "You need a companion with at least 50 cunning to disarm the first trap." Workaround: I was able to disarm all the traps in the area with Isabela at only 32 cunning and the "Bonus to disarm traps" item, Thrice-Bound. A rogue Hawke with 40+ cunning and Boots of Enasalin can disarm the traps as well.
After destroying or reading the grimoire (which can grant you +2 attribute points), you will confront Xebenkeck, a powerful desire demon. The boss will summon hordes of rage demons and abominations, as well as a revenant in the second wave. You will gain 730 XP for defeating them. See the article on Xebenkeck for suggested strategies.
Damage: 41 fire damage (56 per second) +117 attack +4 fire damage across 2m Blood Magic: Each point of health provides 0.25 additional mana, looted from Xebenkeck